JPWO2003064979A1 - Ultrasonic transceiver and ultrasonic flowmeter - Google Patents

Abstract

The ultrasonic transmitter / receiver of the present invention includes a piezoelectric body 4 that performs ultrasonic vibration, a density of 50 kg / m 3 or more and 1000 kg / m 3 or less, and an acoustic impedance of 2.5 × 103 kg / m 2 / s or more and 1.0 ×. An acoustic matching layer 3 formed of a material of 106 kg / m 2 / s or less, a lower acoustic matching layer 9 provided between the piezoelectric body 4 and the acoustic matching layer 3, a lower acoustic matching layer 9 and the piezoelectric body 4 And a structural support body 6 that supports and shields the piezoelectric body 4 from the ultrasonic wave propagation fluid. A protective portion that contacts at least a part of the side surface of the acoustic matching layer 4 is provided. The protective portion is formed by a part of the lower acoustic matching layer 9 and is integrated with the lower acoustic matching layer 9.

Description

逆に、超音波送受信器１０１ｂを超音波送波器として、超音波送受信器１０１を超音波受波器として用いたときのシング・アラウンド周期をｔ２、シング・アラウンド周波数ｆ２とすれば、次式（２）の関係が成立する。

両シング・アラウンド周波数の周波数差Δｆは、次式（３）で示される。

式（３）によれば、超音波の伝搬経路の距離Ｌと周波数差Δｆとから、流体の流速Ｖを求めることができる。そして、その流速Ｖから、流量を決定することができる。
このような超音波流量計では、高い精度が求められる。精度を高めるには、超音波送受信器内の圧電体の超音波送受信面に形成される音響整合層の音響インピーダンスが重要となる。音響整合層は、特に、超音波送受信器が気体に超音波を放射（送波）する場合、および、気体を伝搬して来た超音波を受け取る場合に重要な役割を果たす。
以下、図３６を参照しながら、音響整合層の役割を説明する。図３６は、従来の超音波送受信器１０３の断面構成を示している。
図示されている超音波送受信機１０３は、センサーケース１０５の内側に固定された圧電体１０６と、センサーケース１０５の外側に固定された音響整合層１０４とを備えている。音響整合層１０４は、エポキシ系の接着剤等によってセンサーケース１０５に接着されている。同様にして、圧電体１０６もセンサーケース１０５に接着されている。
圧電体１０６の超音波振動は、接着層を介してセンサーケース１０６に伝わり、更にもうひとつの接着層を介して音響整合層１０４に伝わる。この後、超音波振動は、音響整合層１０４と接する気体（超音波伝搬媒体）に音波として放射される。
音響整合層１０４の役割は、圧電体の振動を効率良く気体に伝搬させることにある。以下、この点をより詳細に説明する。
物質の音響インピーダンスＺは、その物質中の音速Ｃと物質の密度ρを用いて、次の式（４）によって定義される。

超音波の放射対象となる気体の音響インピーダンスは、圧電体の音響インピーダンスから大きく異なっている。一般的な圧電体であるＰＺＴ（チタン酸ジルコン酸鉛）等のピエゾセラミックスの音響インピーダンスＺ１は、３０×１０^６ｋｇ／ｍ^２／ｓ程度である。これに対して、空気の音響インピーダンスＺ３は４００ｋｇ／ｍ^２／ｓ程度である。
音響インピーダンスが異なる物質の境界面では、音波が反射しやすく、境界面を透過する音波の強度が低下する。このため、圧電体と気体との間に、式（５）で示す音響インピーダンスＺ２を持つ物質を挿入することが行われている。

このような音響インピーダンスＺ２を持つ物質を挿入すると、境界面での反射が抑えられ、音波の透過率が向上する。
音響インピーダンスＺ１を３０×１０^６ｋｇ／ｍ^２／ｓ、音響インピーダンスＺ３を４００ｋｇ／ｍ^２／ｓとした場合、式（５）を満たす音響インピーダンスＺ２は、１１×１０^４ｋｇ／ｍ^２／ｓ程度となる。１１×１０^４ｋｇ／ｍ^２／ｓの値を持つ物質は、当然に、式（４）、すなわち、Ｚ２＝ρ×Ｃを満足しなければならない。このような物質を固体材料の中から見出すことは極めて難しい。その理由は、固体でありながら、密度ρが充分に小さく、かつ、音速Ｃが低いことが要求されるからである。
現在、音響整合層の材料としては、ガラスバルーンやプラスチックバルーンを樹脂材料で固めた材料が広く用いられている。また、このような音響整合層に過した材料を作製する方法として、中空ガラス球を熱圧縮する方法や溶融材料を発泡させる方法などが、例えば特許第２５５９１４４号公報に開示されている。
しかし、これらの材料の音響インピーダンスは、５０×１０^４ｋｇ／ｍ^２／ｓより大きい値であり、式（５）を満足しているとは言い難い。高感度な超音波送受信器を得るためには、音響インピーダンスを更に小さくした材料で音響整合層を形成することが必要である。
このような要望に応えるため、本願出願人は、式（５）を充分に満足する音響整合材料を発明し、特願平２００１−０５６０５１号の明細書に開示している。この材料は、耐久性を付与した乾燥ゲルを用いて作製され、密度ρが小さく、かつ、音速Ｃも低い。
このような音響インピーダンスの極めて低い乾燥ゲルなどの材料から形成した音響整合層を備えた超音波送受信器は、気体との間で効率的かつ高感度で超音波の送受信を行うことができる。その結果、気体の流量を高い精度で測定することのできる装置が実現する。
しかしながら、乾燥ゲルのような音響インピーダンスが極めて低い材料は、一般に機械的強度が低い。特に、乾燥ゲルは圧縮方向の応力には比較的強いが、引張りや曲げ方向の応力に極めて弱く、弱い衝撃によっても容易に破壊されてしまう。
また、このような材料は音速が非常に遅いため、最大の送受信惑度を得るための適切な音響整合層の厚さ（送受信波長の約１／４）が非常に薄くなる。例えば、材料の音速が６０〜４００ｍ／ｓであれば、５００ｋＨｚ程度の超音波の送受信を行う場合、音響整合層の好ましい厚さは３０〜２００μｍ程度となる。このように薄くなると、音響整合層をひとつの部材として取り扱うことか極めて困難であり、センサーケースや圧電体に対して音響整合層を接着して、超音波送受信器を作製することが殆ど不可能であるか、可能であっても製造歩留まりやコストの観点から実用化を難しくしている。
更に、音響整合層の機械的な強度が低いことにより、超音波送受信器として使用中に超音波振動自体が音響整合層の剥がれを誘発するなどして、信頼性が低下する可能性もある。
本発明は、上記課題に鑑みてなされたものであり、その目的とするところは、乾燥ゲルなどの機械的強度が低く、音速の遅い材料から形成した音響整合層を備えながら、歩留まり良く製造でき、かつ信頼性の高い超音波送受信器およびその製造方法を提供することにある。
本発明の他の目的は、上記の超音波送受信器を備えた超音波流量計を提供することにある。
発明の開示
本発明の超音波送受信器は、圧電体と、前記圧電体上に設けられた音響整合層と、前記音響整合層の側面の少なくとも一部に接触し、前記圧電体に対して固定された位置に設けられている保護部とを備えている。
好ましい実施形態において、前記保護部は、前記圧電体の主面と同一レベルの平面から超音波放射方向に突出しており、前記圧電体の主面を基準とする前記保護部の高さが前記音響整合層の厚さを規定している。
好ましい実施形態において、前記保護部の前記高さは、５μｍ以上２５００μｍ以下である。
好ましい実施形態において、前記音響整合層の厚さは、前記保護部の前記高さに略等しい。
好ましい実施形態において、前記音響整合層の厚さは、前記圧電体によって送信および／または受信される超音波の波長の約１／４である。
好ましい実施形態において、前記音響整合層は、密度が５０ｋｇ／ｍ^３以上１０００ｋｇ／ｍ^３以下の材料から形成されている。
好ましい実施形態において、前記音響整合層は、音響インピーダンスが２．５×１０^３ｋｇ／ｍ^２／ｓ以上１．０×１０^６ｋｇ／ｍ^２／ｓ以下の材料から形成されている。
好ましい実施形態において、前記音響整合層は、無機系材料から形成されている。
好ましい実施形態において、前記無機系材料は、無機酸化物の乾燥ゲルである。
好ましい実施形態において、前記無機酸化物は、撥水化された固体骨格部を有している。
好ましい実施形態において、前記音響整合層は、前記保護部が設けられた前記圧電体上で流動性状態から固体化されたものである。
好ましい実施形態において、前記圧電体の主面と前記音響整合層との間に設けられた下層音響整合層を備えており、前記保護部は、前記第２の音響整合層の主面から突出しており、前記音響整合層の主面を基準とする前記保護部の高さが最上層に位置する前記音響整合層の厚さを規定している。
好ましい実施形態において、前記保護部は、前記下層音響整合層の一部によって構成され、前記下層音響整合層と一体化している。
好ましい実施形態において、前記保護部の前記高さは、５μｍ以上２５００μｍ以下である。
好ましい実施形態において、前記保護部の前記高さは、最上層に位置する前記音響整合層の厚さに略等しい。
好ましい実施形態において、前記第１の音響整合層および前記下層音響整合層は、それぞれ、前記圧電体によって送受信される超音波の波長の約１／４の厚さを有している。
好ましい実施形態において、前記第１の音響整合層の密度は、５０ｋｇ／ｍ^３以上１０００ｋｇ／ｍ^３以下である。
好ましい実施形態において、前記下層音響整合層の音響インピーダンスは、前記第１の音響整合層の音響インピーダンスよりも大きく、２．５×１０^３ｋｇ／ｍ^２／ｓ以上３．０×１０^７ｋｇ／ｍ^２／ｓ以下である。
好ましい実施形態において、前記保護部は最上層に位置する前記音響整合層の外周に存在している。
好ましい実施形態において、前記保護部は最上層に位置する前記音響整合層の外周側面の全体を覆っている。
好ましい実施形態において、前記保護部は、前記圧電体の主面の外側に配置されている。
好ましい実施形態において、前記保護部は、前記圧電体の主面上に設けられている。
好ましい実施形態において、前記保護部は、前記下層音響整合層上に設けられている。
好ましい実施形態において、前記保護部は、前記下層音響整合層の一部によって構成され、前記下層音響整合層と一体化している。
好ましい実施形態において、前記圧電体を支持する構造支持体を更に備えている。
好ましい実施形態において、前記圧電体を支持する構造支持体を更に備えており、前記保護部が前記構造支持体上に設けられている。
好ましい実施形態において、前記構造支持体はプレス成形された金属から形成されており、前記保護部は、前記構造支持体のプレス成形により折り曲げられた部分によって構成されている。
好ましい実施形態において、前記保護部の前記高さは、５μｍ以上２５００μｍ以下である。
好ましい実施形態において、前記音響整合層の厚さは、前記保護部の前記高さに略等しい。
好ましい実施形態において、前記音響整合層の厚さは、前記圧電体によって送受信される超音波の波長の約１／４である。
好ましい実施形態において、前記音響整合層の密度は、５０ｋｇ／ｍ^３以上１０００ｋｇ／ｍ^３以下である。
前記音響整合層の音響インピーダンスは、２．５×１０^３ｋｇ／ｍ^２／ｓ以上１．０×１０^６ｋｇ／ｍ^２／ｓ以下である。
好ましい実施形態において、前記圧電体の背面に配置された背面負荷材を更に備えており、前記保護部材は前記背面負荷材上に設けられている。
好ましい実施形態において、前記保護部は、前記背面負荷材の一部によって構成されており、前記背面負荷材と一体化している。
好ましい実施形態において、前記音響整合層が前記保持部と接触する面の少なくとも一部は、水酸基を付与する表面処理を受けている。
好ましい実施形態において、前記音響整合層が前記保持部と接触する面の少なくとも一部は、粗面化処理を受けている。
好ましい実施形態において、前記音響整合層が前記保持部と接触する面の少なくとも一部は、多孔質である。
好ましい実施形態において、超音波送受信器のうち前記音響整合層が接触している部分には、前記音響整合層の一部が浸透し、一体化している。
本発明の他の超音波送受信器は、超音波振動を行う圧電体と、密度が５０ｋｇ／ｍ^３以上１０００ｋｇ／ｍ^３以下で、かつ、音響インピーダンスが２．５×１０^３ｋｇ／ｍ^２／ｓ以上１．０×１０^６ｋｇ／ｍ^２／ｓ以下の材料から形成された上層音響整合層と、前記圧電体と前記上層音響整合層との間に設けられた下層音響整合層と、前記下層音響整合層および前記圧電体を支持し、前記圧電体を超音波伝搬流体から遮蔽する構造支持体とを備えた超音波送受信器であって、前記上層音響整合層の側面の少なくとも一部に接触する保護部を備えている。
前記保護部は、前記下層音響整合層の一部によって形成されており、前記下層音響整合層と一体化している。
好ましい実施形態において、前記保護部の弾性率は、前記音響整合層の弾性率に略等しい。
本発明の超音波流量計は、被測定流体が流れる流量測定部と、前記流量測定部に設けられ、超音波信号を送受信する一対の超音波送受信器と、前記一対の超音波送受信器の間を超音波が伝搬する時間を計測する計測手段と、前記計測手段からの信号に基づいて流量を算出する流量演算手段とを備えた超音波流量計であって、前記一対の超音波送受信器の各々が上記いずれかの超音波送受信器である。
好ましい実施形態において、前記超音波送受信器の圧電体は、前記被測定流体から遮蔽されている。
本発明の装置は、上記いずれかの超音波送受信器を備えていることを特徴とする。
本発明による超音波送受信器の製造方法は、主面と、前記主面上に受けられた凸部とを有する圧電体を用意する工程（ａ）と、前記圧電体の主面上に音響整合層を形成し、前記音響整合層の側面の少なくとも一部を前記凸部の側面に接触させる工程（ｂ）とを含んでいる。
好ましい実施形態において、前記工程（ｂ）は、ゲル原料を前記圧電体素子の主面上に供給する工程と、前記ゲル原料を乾燥させ、固化することによって前記音響整合層を形成する工程と、
を含んでいる。
好ましい実施形態において、前記工程（ａ）は、圧電体の表面を加工し、前記主面および凸部を形成する工程を含んでいる。
好ましい実施形態において、前記工程（ａ）は、圧電体の表面に前記凸部を固着する工程を含んでいる。
好ましい実施形態において、前記工程（ａ）は、圧電体を構造支持体に固着する工程を含んでいる。
本発明の超音波送受信器の製造方法は、上層音響整合層、圧電体、および、前記上層音響整合層と前記圧電体との間に設けられた下層音響整合層を備えた超音波送受信器を製造する方法であって、凹部を有し、前記下層音響整合層として機能することになる部材を用意する工程（ａ）と、前記部材の凹部に、ゲル原料を供給する工程（ｂ）と、前記ゲル原料を乾燥させ、固化することによって前記上層音響整合層を形成する工程（ｃ）とを含んでいる。
好ましい実施形態において、前記工程（ｂ）は、前記部材に前記ゲル原料を浸透させる工程を含んでいる。
好ましい実施形態において、前記ゲル原料は前記部材の全体に浸透させられる。
好ましい実施形態において、前記工程（ｂ）は、前記部材と前記圧電体との配置関係を固定した後に行う。前記工程（ｂ）は、前記部材と前記圧電体との配置関係を固定する前に行ってもよい。
本発明の超音波送受信器は、圧電体と、前記圧電体上に設けられた音響整合層と、前記音響整合層の外周面に接するように配置された保護部とを備えている。
本発明の超音波送受信器は、構造支持体と、前記構造支持体を挟んで対抗する位置に設けられた圧電体および音響整合層と、前記音響整合層の外周面に接するように配置された保護部とを備えている。
本発明の更に他の超音波送受波器は、超音波の送波および／または受波を行う主面を有する圧電体と、前記圧電体の主面上に設けられた音響整合部材とを備えた超音波送受波器であって、前記音響整合部材は、第１音響整合部分と、前記第１音響整合部分の平均密度よりも低い平均密度を有する第２音響整合部分とを有しており、前記第１音響整合部分は、前記第２音響整合部分の側面と接触している。
好ましい実施形態において、前記第１音響整合部は、前記第２音響整合部よりも厚く、前記圧電体の主面から放射され前記第２音響整合部分を透過して前記第１音響整合部分の上面と同一レベルの位置に到達した超音波の位相と、前記主面から放射され前記第１音響整合部分を透過して前記第１音響整合部分の上面に到達した超音波の位相とが略一致している
好ましい実施形態において、前記第１音響整合部分における超音波の波長をλ１としたとき、前記第１音響整合部分の厚さはｋ１・λ１の大きさ（ｋ１は１／８以上１／３以下）を有し、かつ前記第２１音響整合部分の厚さとは異なっている。
好ましい実施形態において、前記第２音響整合部分は、Ｎ層の音響整合層（Ｎは１以上の整数）から構成されており、Ｎ層の音響整合層の各々は、各音響整合層における前記超音波の波長のｋ２倍の大きさ（ｋ２は１／８以上１／３以下）を有している。
好ましい実施形態において、前記第２音響整合部分の最外層に位置する音響整合層の厚さは、前記第２音響整合部分の最外層に位置する音響整合層における超音波の波長の約１／４である。
好ましい実施形態において、前記第２音響整合部分のうち、前記圧電体の主面に最も近い位置に形成されている音響整合層は、前記第１音響整合部分の材料と同じ材料から構成されている。
好ましい実施形態において、前記第２音響整合部分のうち、前記圧電体の主面に最も近い位置に形成されている音響整合層は、前記第１音響整合部分と一体的に形成されている。
好ましい実施形態において、前記第２音響整合部分に含まれる少なくとも１層の音響整合層は、乾燥ゲルから形成されている。
好ましい実施形態において、前記乾燥ゲルは、無機系材料からなる。
好ましい実施形態において、前記乾燥ゲルは、撥水化された固体骨格部を有している。
好ましい実施形態において、音波送受波器を構成する部材のうち、前記音響整合部分に接する面の少なくとも一部が、粗面または多孔質である。
好ましい実施形態において、超音波送受波器を構成する部材のうち、前記第２音響整合部分に接する面の少なくとも一部において、前記第２音響整合部分の一部が前記部材に浸透一体化している。
好ましい実施形態において、前記第２音響整合部分の少なくとも一部は乾燥ゲルから形成されており、前記第１音響整合部分は前記乾燥ゲルよりも機械的強度の高い材料から形成されている。
好ましい実施形態において、前記第１音響整合部分の少なくとも一部は、多孔質セラミックスから形成されている。
好ましい実施形態において、前記第１音響整合部分の厚さは、前記圧電体の主面における位置に応じて変化している。
好ましい実施形態において、前記第２音響整合部分の厚さは、前記圧電体の主面における位置に応じて変化している。
本発明の超音波流量計は、被測定流体が流れる流量測定部と、前記流量測定部に設けられ、超音波信号を送受波する一対の超音波送受波器と、前記一対の超音波送受波器の間を超音波が伝搬する時間を計測する計測部と、前記計測部からの信号に基づいて流量を算出する流量演算部とを備えた超音波流量計であって、前記一対の超音波送受波器の各々が、上記いずれかの超音波送受波器である。
好ましい実施形態において、前記超音波送受波器の圧電体は、前記被測定流体から遮蔽されている。
好ましい実施形態において、前記被測定流体は、気体である。
本発明の装置は、上記いずれかの超音波送受波器を備えている。
本発明の超音波送受波器の製造方法は、（ａ）第１の面および前記第１の面とは反対側の第２の面を有し、前記第１および第２の面に電極が形成された圧電体を用意する工程と、（ｂ）前記圧電体における前記第１および第２の面の少なくとも一方の側に第２音響整合部分を形成する工程と、（ｃ）前記圧電体と前記第２音響整合部分とによって形成された空間内にゲル原料を供給する工程と、（ｄ）前記ゲル原料液をゲル化させて湿潤ゲルを得る工程と、（ｅ）得られた湿潤ゲルを乾燥させる工程とを含む。
好ましい実施形態において、前記工程（ｃ）は、（ｃ１）前記空間内に第１のゲル原料を供給する工程と、（ｃ２）前記第１のゲル原料液をゲル化させて第１の湿潤ゲル層を形成する工程と、（ｃ３）前記第１の湿潤ゲル層の上に第２のゲル原料を供給する工程と、（ｃ４）前記第２のゲル原料液をゲル化させて第２の湿潤ゲル層を形成する工程とを含み、前記工程（ｅ）は、前記第１および第２の湿潤ゲル層を乾燥させることにより、前記第１および第２の湿潤ゲル層から、それぞれ、第１音響整合層および第２音響整合層を形成する工程とを含む。
好ましい実施形態において、前記工程（ｃ４）において、前記第１音響整合層の音響インピーダンスを変化させるように前記第１の湿潤ゲル層を改質する。
本発明の超音波送受波器は、超音波の送波および／または受波を行う主面を有する圧電体と、前記圧電体の主面上に設けられた音響整合部材とを備えた超音波送受波器であって、前記音響整合部材は、第１音響整合部分と、前記第１音響整合部分の機械強度よりも低い機械強度を有する第２音響整合部分とを有しており、前記第１音響整合部分は、前記第２音響整合部分の側面と接触している。
発明を実施するための最良の形態
以下、図面を参照しながら、本発明の実施形態を説明する。
（実施形態１）
図１は、本発明の第１の実施形態における超音波送受信器（超音波振動子）の一断面を示している。図示されている超音波送受信器１は、圧電体４と、圧電体４上に設けられた音響整合層３と、圧電体４に対して固定された保護部２とを備えている。
圧電体４は、圧電性を有する材料から形成され、厚さ方向に分極されている。圧電体４の上下面には、不図示の電極が形成され、電極に印加される信号に基づいて超音波を放射する。また、超音波を受けた場合は、電極間に電圧信号を発生させる。本発明では、圧電体４の材料は任意であり、公知のものを用いることができる。
圧電体４の主面（超音波送受波面）Ｓ１を基準とする保護部２の高さＨは音響整合層３の厚さを規定しており、好ましい態様では、保護部２の高さは音響整合相層３の厚さに略等しい。
図２は、図１の超音波送受信器１の上面を示している。図２からわかるように、本実施形態の超音波送受信器１では、リング状の保護部２が音響整合層３を取り囲み、音響整合層３の外周面（側面）の全体が保護部２の内周面と接触している。このような保護部２を圧電体４の上面に設けることにより、圧電体４から音響整合層３が剥離しにくくなり、また、音響整合層３の破損を防止することができる。その結果、超音波送受信器１の製造段階および使用段階における信頼性が著しく向上する。
なお、のちに述べる製造方法によれば、保護部２の高さＨを調節することによって音響整合層３の厚さを高精度で制御することができる。そのため、音響整合層３を高い精度で安定的に形成することができるので、品質に優れた超音波送受信器を歩留りよく製造することが可能となる。音響整合層３の厚さが素子ごとにばらつくと、超音波送受信器の特性（感度など）が変動するため、所定の厚さを有する音響整合層３を再現性良く形成することが重要である。前述したように、最大の送受信感度を得るための適切な音響整合層の厚さは、送受信する超音波の波長の約１／４である。このため、音速が約２８０ｍ／ｓの乾燥ゲルを音響整合層に用いて５００ｋＨｚ程度の超音波の送受信を行う場合は、乾燥ゲルの音響整合層の好ましい厚さを１４０μｍ程度に設定する必要がある。この厚さが１０％程度異なると、送受信感度は２０％程度変動するおそれがある。このように音響整合層３の厚さが僅かに変化するだけで、送受信感度が大きく変動するが、本実施形態によれば、所望の厚さを有する音響整合層３が再現性良く形成されている。
本実施形態の超音波送受信器１は、例えば、以下のようにして製造される。
まず、送受信する超音波の波長に合わせた圧電体４を用意する。圧電体４としては、圧電セラミックスや圧電単結晶など圧電性の高い材料が好ましい。圧電セラミックスとしては、チタン酸ジルコン酸鉛、チタン酸バリウム、チタン酸鉛、ニオブ酸鉛などを用いることができる。また圧電単結晶としては、チタン酸ジルコン酸鉛単結晶、ニオブ酸リチウム、水晶などを用いることができる。
本実施形態では、圧電体４としてチタン酸ジルコン酸鉛圧電セラミックスを用い、送受信する超音波の周波数を５００ｋＨｚに設定している。このような超音波を圧電体４が効率よく送受信できるようにするため、素子の共振周波数を５００ｋＨｚに設計する。このため、本実施形態では、直径が１２ｍｍ、厚さが約３ｍｍの円柱形状を有する圧電セラミックスから形成された圧電体４を用いている。
このような圧電体４に対して、外径１２ｍｍ、内径１１ｍｍ、厚さ１４０μｍのリング状保護部２を接合する。本実施形態では、保護部２として、ステンレス製の金属リングを用いている。ステンレス性の保護部２と圧電体４との接合は、接着剤による接着によって行うことができる。例えば、接着剤としてエポキシ系樹脂を用い、０．２ＭＰａの圧力をかけながら、１５０℃の恒温槽中で、２時間放置して硬化させればよい。
本実施形態では、乾燥ゲルから音響整合層３を形成する。乾燥ゲルから形成した音響整合層３の音速は約２８０ｍ／ｓであるため、音響整合層３における超音波の波長は約６４０μｍである。このため、音響整合層３の厚さを、音響整合層３における超音波波長の約１／４に等しくなるように、１４０μｍに設定している。この厚さの音響整合層３を形成するため、本実施形態では、保護部２の厚さを１４０μｍに設定している。
保護部２の役割は、まず第１に、超音波送受信器１の製造段階や使用段階における外部から受ける機械的な衝撃または熱的な衝撃から、音響整合層３を保護することにある。第２に、超音波送受信器１としての動作（使用）時に、送受信する超音波の振動から超音波送受信器１を保護することも重要な役割である。
音響整合層３が、その役割を果たすためには、圧電体４と音響整合層３とが密着していることが極めて重要である。圧電体４と音響整合層３との間に僅かでも剥離が生じると、音響整合層３としての役割を果たすことができなくなる。
本願発明者は、圧電体４と音響整合層３との密着性を保持するためには、図２に示すように、音響整合層３の外周部分に保護部２を設けた構造が極めて有効であることを見いだした。保護部２が無い場合には、超音波送受信器１の製造時や使用時に大幅な特性劣化が進行し、高性能の超音波送受信器１を実用化することができなくなる可能性がある。
本実施形態の音響整合層３は、密度ρと音速Ｃとの積（ρ×Ｃ）で規定される音響インピーダンスが極めて小さい材料から形成される。このため、空気などの気体に対する超音波の送受信効率を極めて高くすることができる。音響インピーダンスが極めて小さい材料として、本実施形態では、前述したように乾燥ゲルを用いている。
音響整合層３を乾燥ゲルから形成することにより、ガラスバルーンやプラスチックバルーンを樹脂材料で固めた従来材料から音響整合層を形成した場合に比較して、気体と圧電体との間の音響整合が極めて良くなるため、超音波送受信効率を格段に向上させることができる。
本明細書における「乾燥ゲル」とは、ゾルゲル反応によって形成される多孔質体であって、ゲル原料液の反応によって固体化した固体骨格部が、溶媒を含んで構成された湿潤ゲルを経て、乾燥して溶媒除去することで形成されたものである。この乾燥ゲルは、ナノメートルサイズの固体骨格部によって平均細孔直径が数ｎｍから数μｍ程度の連続気孔が形成されているナノ多孔質体である。
このような微細な構造を有する多孔質体であるため、固体部分を伝搬する音速が極端に小さくなるとともに、細孔によって多孔質体内の気体部分を伝搬する音速も極端に小さくなるという性質を有する。そのため、音速として５００ｍ／ｓ以下程度の非常に遅い値を示し、従来の音響整合層とは全く異なる低い音響インピーダンスを得ることができる。また、ナノメートルサイズの細孔部では、気体の圧損が大きいために音響整合層として用いた場合に、音波を高い音圧で放射できるという特徴も有する。
このような乾燥ゲルの材質としては、無機材料、有機高分子材料など様々な材料を用いることができる。無機材料の固体骨格部としては、酸化ケイ素（シリカ）、酸化アルミニウム（アルミナ）、酸化チタンなどを用いることができる。また有機材料の固体骨格部としては、一般的な熱硬化性樹脂、熱可塑性樹脂を用いることができ、例えば、ポリウレタン、ポリウレア、フェノール硬化樹脂、ポリアクリルアミド、ポリメタクリル酸メチルなどを用いることができる。
本実施形態では、予め圧電体４と保護部２とによって形成された凹型空間Ｐ１（図１参照）の内部に上述の乾燥ゲルから音響整合層３を形成する。すなわち、液体状であるゲル原料液を圧電体４と保護部２とで構成される凹型空間Ｐ１に流し込んだ後、ゲル化、疎水化、および乾燥を行うことにより、音響整合層３となる乾燥ゲルを形成する。なお、本実施形態では、酸化ケイ素の固体骨格部を有する乾燥ゲルを音響整合層３として用いている。
具体的には、以下に示す工程１〜４を順次行うことにより、音響整合層３を形成することができる。
１、テトラエトキシシラン、エタノール、およびアンモニア水溶液（０．１規定）を、モル比で、１：３：４となるように調製したゲル原料液（ゾル）を用意する。
２、このゲル原料液をスポイトで圧電体と保護部で形成される凹型空間に滴下する。そのとき、凹型空間Ｐ１の体積よりも過剰な量のゲル原料液を滴下する。次に、凹型空間Ｐ１の内部に溜まったゲル原料液の高さが保護部の高さＨと同じになるよう、テフロン（登録商標）製の平板（不図示）を用いたすり切り操作を行った後、テフロン板で蓋をする。
３、室温で約１日放置し、原料液がゲル化（湿潤ゲルの形成）した後、テフロン板を取り外す。その後、トリメチルエトキシシランの５重量％ヘキサン溶液中で、疎水化処理を行う。
４、超臨界乾燥槽に導入し、二酸化炭素雰囲気のもと、１２ＭＰａ、５０℃の条件で超臨界乾燥をおこなう。こうして、乾燥ゲルが形成される。
このような工程１〜４により、例えば、密度ρが０．３×１０^３ｋｇ／ｍ^３、音速Ｃが２８０ｍ／ｓ、厚さが１４０μｍの音響整合層４を形成することができる。
本発明は、密度が５０ｋｇ／ｍ^３以上１０００ｋｇ／ｍ^３以下で、かつ、音響インピーダンスが２．５×１０^３ｋｇ／ｍ^２／ｓ以上１．０×１０^６ｋｇ／ｍ^２／ｓ以下の材料から音響整合層を形成する場合に顕著な効果を発揮するが、上記の方法によれば、このような音響整合層を好適に作製することが可能となる。
上記方法によれば、音響整合層３の厚さを保護部２の高さＨに略等しくすることができるため、保護部２によって音響整合層３の厚さを高精度で制御できる。保護部２は、製造工程のある段階においては、ゲル原料液のガイドとして機能しているといえる。
上記方法によれば、厚さばらつきの少ない音響整合層３を歩留まり良く形成できるため、超音波送受信器の特性ばらつきを抑制することが可能になる。なお、本発明において、保護部２の高さを音響整合層３の厚さと等しくすることは不可欠ではない。保護部２の高さが音響整合層３の厚さより大きい場合は、音響整合層３の収縮を抑制し、機械的な衝撃から保護する機能は充分に発揮される。逆に、保護部２の高さが音響整合層３の厚さより小さい場合でも、保護部２を設けない場合にくられべれば、音響整合層３の収縮を抑制し、機械的な衝撃から保護する機能が高い程度で発揮される。
上記方法で作製した音響整合層４を備えた超音波送受信器について、その送受信波形を測定した。測定により得られた波形図を図３（ａ）に示す。比較のため、ガラスバルーンをエポキシで固めた材料を音響整合層に用いた場合の送受信波形を図３（ｂ）に示す。ここで用いたガラスバルーンの音響整合層は、密度が０．５２ｇ／ｃｍ^３、音速が２５００ｍ／ｓ、厚さが１．２５ｍｍである。
なお、従来技術について説明したように、音響整合層の音響インピーダンスは、式（５）で規定される値を示すことが望ましい。本実施形態では、圧電体４にチタン酸ジルコン酸鉛圧電セラミックスを用い、超音波を伝搬する伝搬媒体としては空気を設定している。従って、圧電体４の密度が７．７×１０^３ｋｇ／ｍ^３であり、音速が３８００ｍ／ｓであるので、音響インピーダンスは約２９×１０^６ｋｇ／ｍ^２／ｓとなる。一方、空気の密度は０．００１１８ｋｇ／ｍ^３であり、その音速は３４０ｍ／ｓであるので、音響インピーダンスは約０．０００４×１０^６ｋｇ／ｍ^２／ｓとなる。このため、式（５）より、音響整合層の好ましい音響インピーダンスは、理論上、約０．１×１０^６ｋｇ／ｍ^２／ｓとなる。
本実施形態の超音波送受信器１における音響整合層３の音響インピーダンスは、密度が０．３×１０^３ｋｇ／ｍ^３で、音速が２８０ｍ／ｓであるため、約０．０８４×１０^６ｋｇ／ｍ^２／ｓとなり、理論上の理想値に極めて近いものとなっている。
図３からわかるように、本実施形態によれば、従来のセンサに比較して３倍以上の送受信感度を得ることができる。また、本実施形態では、保護部２を設けているため、機械的強度が低く壊れやすい乾燥ゲルから形成した音響整合層を有する超音波送受信器でも、歩留まり良く製造され、しかも、使用時においても、長期間信頼に足る動作を継続することができる。外部振動テスト、熱的衝撃のテスト、連続振動テストなどを行い、音響整合層３が圧電体４から剥離するか否かを厳しい条件下で評価したが、超音波送受信器の性能が劣化することは無く、極めて安定な動作を確認することができた。
本実施形態では、上記工程４において、湿潤ゲルを乾燥させて乾燥ゲルを得る際、超臨界乾燥法を用いたが、通常の大気中での乾燥を行っても良い。この場合、湿潤ゲルから乾燥ゲルへの変化する過程で収縮が起こり、１０〜２０％程度の体積変化が発生する。このような体積収縮が生じると、従来の構成では、音響整合層３が圧電体４から剥離する。しかし、本実施形態の場合は、図４に示すように、保護部２が存在するため、乾燥ゲルの収縮が主として厚さ方向にのみ起こる。すなわち、圧電体４と音響整合層３との界面で、面内方向の応力がほとんど発生せず、音響整合層３の剥離が効果的に防止される。従って、超臨界乾燥法よりも簡便な大気中乾燥法を採用しても、高感度・高信頼性の超音波送受信器１を作製することができ、製造コストを低減することが可能となる。
なお、保護部２の高さは、乾燥ゲルの収縮率を考慮して、最終的な音響整合層３の厚さが最適な大きさを持つように設定されることが好ましい。なお、収縮の程度が大きくなりすぎて、音響整合層３の最も薄い部分の厚さが平均厚さの９０％以下に減少すると、音響整合層３の特性が劣化するので好ましくない。超臨界乾燥法によれば、音響整合層３の最も薄い部分の厚さを平均厚さの９８％以上に維持することができる。
音響整合層３の下面と接する圧電体４の上面、および音響整合層３の側面と接する保護部２の内側面に対しては、前もって、プラズマクリーニングや酸処理などの表面処理を行っておくことが好ましい。このような処理によって接触面に水酸基を形成しておくと、乾燥ゲルと圧電体４および保護部２との間の化学的な結合をより強固にすることができる。
音響整合層３と圧電体４および保護部２と間で強固な結合を実現するには、圧電体４および保護部２の表面のうち、音響整合層３に接する領域を粗面化してもよい。粗面化の手法としては、通常のヤスリがけや、ブラスト処理、物理的あるいは化学的なエッチング操作などが有用に利用することができる。
音響整合層３と保護部２との密着性を向上させるには、保護部２の材料として多孔質材料を用いることも有効である。多孔質体から保護部２を形成することにより、音響整合層３の一部が保護部２の内部まで浸透して一体化するため、更に強固な密着状態を得ることができる。
保護部２に使用可能な多孔質体としては、例えば、発泡法等によって製造された金属、セラミック、樹脂などがあげられる。多孔質金属としては、ステンレス、ニッケル、銅など、セラミックとしてはアルミナ、チタン酸バリウムなど、樹脂としてはエポキシ、ウレタンなど様々な材料を用いることもできる。
なお、本明細書において、「音響整合層を保護する」とは、音響整合層を機械的な振動や衝撃から守ることだけではなく、形成時に収縮する材料から音響整合層を作製する工程で音響整合層の剥離を抑制することをも含むものとする。音響整合層をこのようにして保護する部材を採用することにより、機械的強度が弱く、収縮性を有する材料から音響整合層を形成しても、音響整合層の機能（音響的な整合により圧電体と超音波の伝搬媒体との間の超音波の送受信を効率的に行えるような働き）を実用レベルで持続させることができる。
（実施形態２）
図５を参照しながら、本発明の第２の実施形態を説明する。
本実施形態では、保護部と圧電体とが一体化されている。具体的には、圧電体５の主面中央部に凹部５ａを形成し、圧電体５の一部５ｂを保護部として用いている。言い換えると、圧電体５の一部５ｂが保護部として機能し、保護部と圧電体が一体的に形成されている。
図５の超音波送受信器は、次のようにして作製される。
まず、分極処理の済んだ圧電体５を用意した後、圧電体５の一方の面（主面）を加工して凹部５ａを形成する。凹部５ａを形成するための加工は、エンドミルやサンドブラストによって行うことができる。凹部５ａの深さは、保護部（５ｂ）の高さに対応する。この後、凹部５ａが形成された面に電極を形成し、圧電体の凹部と反対側の面にも電極を形成する。電極は、例えば、めっきやスパッタなどにより、金、ニッケルなどの金属膜を形成することによって作製される。
本実施形態によれば、圧電体５を加工し、その主面の周辺部を保護部と機能させるため、別途作製した保護部を圧電体に接合する工程が不要となる。保護部の接合に接着を用いた場合は、接着層の存在による保護部の高さの変化を考慮する必要があるが、本実施形態によれば、高さが高い精度で規定された保護部によって音響整合層３の厚さを高精度で調節できるため、安定して高性能の超音波送受信器を提供することができる。
本実施形態においても、圧電体５の表面のうち、音響整合層３と接触する部分に対して水酸基を形成する処理を行うことが好ましい。また、圧電体５に凹部５ａを形成する加工に際して、圧電体５を粗面化すれば、音響整合層３と圧電体５との密着性を更に向上させることができる。
上記の実施形態１および実施形態２では、保護部として機能する部分は、圧電体５の主面に垂直な側面を有するリング状に形成されている。しかし、この保護部の側面は、図６に示すように、テーパを有していても良い。また、図７に示すように、音響整合層３の外周側面の全てに接触している必要は無く、複数の部分に分割された構造や、一部に切り欠きが形成された構造を有していても良い。
上記の実施形態１または２の構成によれば、乾燥ゲルなどの密度が低く、音速の遅い材料を音響整合層に用いた場合でも、保護部が音響整合層と圧電体との結合を強固にして、高い送受信感度を発揮するとともに、超音波送振動子の製造工程段階における取り扱いを容易にし、高性能の超音波送振動子を高歩留まりで提供することが可能となる。また、超音波送振動子の使用段階における機械的衝撃や、超音波の送受信に伴う振動によっても特性の劣化しにくく信頼性に優れた素子が実現する。
（実施形態３）
図８を参照しながら、本発明の第３の実施形態を説明する。
本実施形態に特徴的な点は、構造支持体６を有している点にある。図８に示す構造支持体６は、音響整合層３なとが固定される円板状支持部６ａと、この円盤状支持部から軸方向に連続的に延びる円筒部６ｂとを備えている。円筒部の端部は、断面がＬ字型に折れ曲がり、遮蔽のためのプレート６０や、他の装置などに固定されやすくなっている。
構造支持体６の支持部６ａの表面には、音響整合層３と保護部２とが配置されており、支持部６ａの裏面には圧電体４が配置されている。すなわち、圧電体４および音響整合３は、それぞれ、構造支持体６を挟んで対向する位置に設けられている。このような構造支持体６を用いることにより、超音波送受信器（超音波送受信器）の取り扱いが極めて容易となる。
構造支持体６を密閉可能な容器（センサーケース）から構成することができる。この場合、構造支持体６の円筒部６ｂの開放端を遮蔽用プレート６０などで塞ぎ、かつ、構造支持体６の内部を不活性ガスで満たせば、流速測定の対象とする流体から圧電体４を遮断することができる。圧電体４には電圧が印加されるため、可燃性ガスで圧電体４が囲まれていると、可燃性ガスに印加する危険性もある。しかし、構造支持体６を密閉型の容器から構成し、内部を外部から遮断することにより、そのような引火を防止できるため、可燃性ガスに対しても安全に超音波を放射することができる。また、外部のガスが可燃性でなくとも、圧電体４と反応し、圧電体４の特性を劣化する可能性のあるガスに超音波を放射する場合でも、圧電体４を外部のガスから遮断することにより、圧電体４の劣化を抑制し、長期間に渡って信頼性の高い動作を実現することが可能となる。
なお、図８の例では、保護部２は、圧電体４の超音波送受信面の外側周辺部に配置されている。一般に保護部２は音響整合層４の役割を果たさないため、圧電体４の主面上に保護部２が配置すると、その部分は超音波の送受信に寄与しない部分となり、送受信感度が低下してしまう。
構造支持体６が音響的阻害要因とならないようにするには、圧電体４が接触する円板状支持部６ａの厚さを、送受信する超音波の波長の１／８以下とすることが望ましい。この厚さを波長の１／８程度以下とすることにより、構造支持体６は超音波の伝搬を阻害しなくなる。
本実施形態では、構造支持体６の材料としてステンレスを用い、構造支持体６の厚さを０．２ｍｍに設定している。ステンレス中の音速は、約５５００ｍ／ｓであるため、０．２ｍｍは周波数５００ｋＨｚの超音波における波長の約５５分の１に相当する。このように薄いステンレスから構造支持体６を形成しているため、構造支持体６が超音波の伝搬経路内に介在しても、殆ど音響的な障害とはならない。
構造支持体６の材料は、金属材料に限定されず、セラミック、ガラス、樹脂なから目的に応じた材料が選択され得る。本実施形態では、外部の流体と圧電体とを確実に分離し、構造支持体６に何らかの機械的衝撃が加わったとしても、圧電体と外部流体との接触を防止することができる強度を与えるため、金属材料から構造支持体６を作製している。これにより、例えば可燃性や爆発性を有するガスを対象として超音波の送受信を行っても、高い安全性を確保することができる。
安全な気体に対して超音波の送受信を行う場合には、コスト低減を目的として、樹脂などの材料から構造支持体６を形成しても良い。
構造支持体６と音響整合層３との密着性を高めるため、構造支持体６の表面のうち、音響整合層３と接触する部分に、前もって、水酸基を付加するプラズマ処理や酸処理を行うことが好ましい。また、やすりがけやサンドブラスト処理、化学的および／または物理的エッチングなどによって、この部分の粗面化を行ってもよい。
（実施形態４）
次に、図９を参照しながら、本発明の第４の実施形態を説明する。
本実施形態の超音波送受信器では、構造支持体７の一部７ａが保護部として機能し、構造支持体７と保護部とが一体化している。例えばステンレスなどの金属材料をプレス成形することによって構造支持体７を作製する際、その円板状支持部に凹部７ｂを形成し、凹部７ｂの周辺（構造支持体７のプレス成形によって折り曲げられた部分７ａ）を保護部として用いることができる。
このような構成を採用することにより、保護部を構造支持体に接合する工程を省くことができる。また、実施形態１と同様に、接着層によって保護部の高さがばらつくこともなくなるため、高感度な超音波送受信器を歩留まり良く作製することができる。
なお、図９では、構造支持体７を密閉するためのプレートを記載していないが、必要に応じて、このようなプレートを構造支持体７に固着または一体化してもよい。以下に説明する他の実施形態でも同様である。
（実施形態５）
図１０および図１１を参照しながら、本発明の第５の実施形態を説明する。
本実施形態の超音波送受信器は、音響整合層３と圧電体４との間に配置された他の音響整合層（下層の音響整合層）８を備えている。下層音響整合層８の挿入を除けば、本実施形態の構成は実施形態２の構成と同様である。
音響整合層は、音響インピーダンスの不整合による音波の内部反射を抑え、効率よく超音波を圧電体から媒体（超音波伝搬媒体）に放射させる役割を果たす。このような音響整合層は、単一の周波数を有する超音波（連続波の超音波）を送信または受信する場合には、１層で充分である。
これに対し、通常の超音波送受信器では、パルスまたはバースト状の超音波を送受信する。パルスまたはバースト状の超音波は、単一の周波数成分ではなく、広帯域の周波数成分を含んでいる。このような超音波の送受信を高感度で行うためには、圧電体と超音波伝搬媒体との間で、音響整合層の音響インピーダンスを徐々に変化されることが好ましい。このように音響インピーダンスを徐々に変化させるには、音響整合層を多層化し、構成層の音響インピーダンスを徐々にシフトさせればよい。
本実施形態では、図１０に示すように、音響整合層を２層化している。具体的には、下層の音響整合層８として、セラミックスからなる多孔質焼結体を用いている。この音響整合層８は、見かけ密度が約０．６４×１０^３ｋｇ／ｍ^３、音速が２０００ｍ／ｓ、音響インピーダンスが約１．２８×１０^６ｋｇ／ｍ^２／ｓである。セラミックスとしては、チタン酸バリウム系の材料を用いている。
「見かけ密度」とは、多孔質体に含まれる空間部分も含んだ密度である。多孔質セラミックスは、体積の約８０％が空間部分（空孔）であり、セラミックスの実体部は全体の約２０体積％である。このような多孔質セラミックスは、樹脂製のボールとセラミックス粉末を混合、加圧成形した後に、セラミックスを焼結させる過程において、樹脂ボールを加熱、燃焼除去することによって形成される。焼結に際して加熱を急激に行うと、樹脂ボールが膨張または急激なガス化を起こし、セラミックス構造体を破壊してしまうため、緩やかな加熱を行うことが好ましい。
本実施形態では、このような下層の音響整合層８を圧電体４（直接的には構造支持体６）に対して固定した後、この音響整合層８のうち、圧電体４とは反対側の面に対して、保護部２を接合する。保護部２は、実施形態１で用いた保護部２と同様にステンレス製のリングから作製したものを用いることができる。接合は全てエポキシ系の接着剤で行うことができる。
こうして下層の音響整合層８と保護部２とによって形成された凹部内に、実施形態１と同様にして、音響整合層３となる乾燥ゲル層を形成した。
本実施形態では、実施形態１における工程１と同様の工程１において、ゲル化反応触媒となるアンモニアの濃度を変更することにより、乾燥ゲルの密度を調整し、音響整合層として密度０．２×１０^３ｋｇ／ｍ^３、音速１６０ｍ／ｓ、音響インピーダンス約０．０３２×１０^６ｋｇ／ｍ^２／ｓの乾燥ゲル層を形成する。保護部２の高さは、音響整合層の音速が１６０ｍ／ｓであるため、音響整合層における超音波波長の１／４となるよう、８０μｍに設定している。保護部２の内側面には、プラズマエッチングにより水酸基を付与する処理を行うことが好ましい。
図１１（ａ）は、本実施形態における超音波送受信器の送受信波形を示している。図１１のグラフ中において、縦軸は信号振幅、横軸は時間である。軸上の数値は指数標記であり、例えば「２．０Ｅ−０４」は２．０×１０^−４を意味している。他のグラフも同様である。
測定に用いた超音波送受信器では、下層の音響整合層８となる多孔質セラミックスの厚さを１ｍｍとし、保護部２および第１音響整合層（乾燥ゲル層）２の厚さを８０μｍに設定した。
比較のため、図１０の超音波送受信器において、２層の音響整合層３、８の代わりに、ガラスバルーンをエポキシで固めた従来の音響整合層を用いた超音波送受信器を作製し、その送受信波形を測定した。測定結果を図１１（ｂ）に示す。
音響整合層を２層とすることにより、従来の超音波センサに比較して約２０倍の高い感度を得ることができた。また。実施形態１における超音波センサと比較しても、高感度化と同時に広帯域化（短パルス化）が図られたことがわかる。このように、音響整合層の２層化により、パルスやバースト波を送受信するのに極めて好適な超音波送受信器を提供することが可能となる。
（実施形態６）
図１２を参照しながら、本発明の第６の実施形態を説明する。
本実施形態では、下層の音響整合層９の一部が保護部として機能し、音響整合層９と保護部とが一体化している。この例では、音響整合層９を加工して、その主面に凹部を形成している。上層の音響整合層３となる乾燥ゲル層は、下層の音響整合層９の凹部内に形成される。
このような構成を採用することにより、保護部を音響整合層９に接合する工程を省くことができる。また、接着層によって保護部の高さがばらつくという問題も解決することができ、広帯域において高感度で動作する超音波送受信器を歩留まり良く製造することができる。
本実施形態では、下層の音響整合層９を多孔質体から形成している。このため、上層の音響整合層３との結合が強く、高感度、高安定性を確保することができる。この密着性を更に高めるため、上層の音響整合層３と下層の音響整合層９との接触面に対して、前もって、水酸基を付与するプラズマ処理または酸処理などを行ってことが好ましい。
本実施形態および前述の実施形態５では、音響整合層が２層構造を有しているが、本発明の超音波振動子は、このような構成に限定されず、３層以上の多層構造を有していてもよい。音響整合層を多層化することにより、更に感度を高め、帯域を広くすることができる。ただし、多層化によって感度を高めるには、音響インピーダンスが極めて低い材料から最外層となる音響整合層を形成する必要があるため、実用上は、２層構造の採用が現実的である。
（実施形態７）
図１３〜１５を参照しながら、本発明の第７の実施形態を説明する。
本実施形態の超音波送受信器は、図１３に示すように、圧電体４の背面側に背面負荷材１０が接合されており、保護部２は背面負荷材１０の上部に形成されている。この他の点では、実施形態３と同様の構成を有している。
背面負荷材１０は、圧電体４から背面側へ放射される超音波を減衰させる機能を有しており、そのような機能を発揮し得る材料であれば、どのような材料から形成されていても良い。
保護部２は、筒状の金属から形成されており背面負荷材１０の主面に接着されている。保護部２の厚さは、圧電体４、下層の音響整合層８、および上層の音響整合層３の合計厚さに等しい。本実施形態では、圧電体４の厚さを３ｍｍ、音響整合層８の厚さを１ｍｍ、音響整合層３の厚さを０．０８ｍｍに設定しているため、保護部２の厚さは４．０８ｍｍである。
本実施形態の背面負荷材１０はフェライトゴムから形成されている。フェライトゴムは、ゴム中に鉄粉を分散させた材料であり、音波の減衰率が高い。このような背面負荷材１０を圧電体４の背面に接合することにより、圧電体４の背面側からで放射された超音波を減衰させ、広帯域の（パルス幅の短い）超音波を送受信することが可能となる。
図１４は、図１３の構成を有する超音波送受信器について測定した送受信波形を示している。実施形態３の超音波送受信器に比べて、本実施形態の送受信感度は低下しているが、より広い帯域での動作が実現し、幅の短いパルスの送受信に適した超音波送受信器を構成することができる。
図１３に示す背面負荷材１０に代えて、図１５に示す背面負荷材１１を用いても良い。図１５に示す背面負荷材１１は、その一部が保護部として機能し、保護部と背面負荷材とが一体化している。背面負荷材１１は、主面の周辺部を除く領域に凹部が形成された構成を有しており、凹部内に圧電体４が挿入され、背面負荷材１１の凹部内面に接着されている。背面負荷材１１に設けられた凹部の深さは、圧電体４の高さよりも大きく設定されており、挿入後の圧電体４の上面に音響整合層となる乾燥ゲル層を形成すれば、図１５の構成が得られる。背面負荷材１１の採用により、図１３の超音波送受信器と同様の広帯域化が達成される。
（実施形態８）
図１６を参照しながら、図１２に示す超音波送受信器の製造方法の実施形態を説明する。
まず、図１６（ａ）に示すように、圧電体４および下層の音響整合層９を構造支持体６に接合する。接合には接着剤を用いることができる。前述のように、圧電体４は圧電セラミックスから形成され、構造支持体６はステンレスから形成されている。下層の音響整合層９は、上面に凹部を有する多孔質セラミックスから形成されている。この凹部は、平板状の多孔質セラミックスの上面を旋盤などによって加工することによって形成される。
次に、図１６（ｂ）に示すように、構造支持体６に接着された状態の音響整合層９の凹部に対し、上層の音響整合層となる乾燥ゲルを形成する。乾燥ゲルの形成は、第１の実施形態について説明した方法で行うことができる。
多孔質セラミックスから形成された音響整合層９にゲル原料を充分に浸透させるため、ゲル原料を流し込んだ後、真空または減圧雰囲気内に配置することが好ましい。このようにして、本実施形態では、ゲル原料を、音響整合層９の保護部として機能する部分だけではなく、音響整合層９の内部全体に浸透させる。こうすることにより、乾燥ゲルを下層の音響整合層９に強固に結合させるとともに、音響整合層９の特性を均一にすることが可能となる（図１６（ｃ））。以下、この点を、図１７を参照して説明する。
図１７（ａ）は、下層の音響整合層９にゲル原料が充分に浸透した状態を示している。このため、下層の音響整合層９は音響的に単一つの層として機能し、音響インピーダンスは、音響整合層９の相対的に高い値から、上層の音響整合層の相対的に低い値へ階段状に減少する。
一方で、ゲル原料の浸透が不十分である場合には、図１７（ｂ）に示すように、実質的に３層構造を音響整合層が形成される。この場合、ゲル原料の浸透が不充分な最下層（第１層）の音響インピーダンスが設定値よりも小さくなるため、真ん中の層（第２層）の音響インピーダンスが最も大きくなる。音響インピーダンスの分布が図１７（ｂ）に示すようになると、図中の下方に配置される圧電体（不図示）から超音波伝搬媒体となる気体に向かって音響インピーダンスが段階的に小さくならず、超音波送受波器の特性が劣化してしまうため、ゲル原料の浸透を充分に行うことが好ましい。
上記の実施形態では、図１６に示すように、下層の音響整合層９を構造支持体６に固定する工程の後、第１音響整合層３を形成する工程を行っているが、これらの工程の順序を反対にしてもよい。図１８（ａ）から（ｄ）を参照して、他の製造方法を説明する。
まず、図１８（ａ）に示すように、保護部として機能する部分を有する音響整合層９を用意する。次に、図１８（ｂ）に示すように、音響整合層９の凹部にゲル原料を滴下し、凹部内のゲル原料の高さを保護部の高さに一致させるようにすり切り、ゲル原料を音響整合層９の全体に浸透させる。ゲル材料の硬化、疎水化の後、超臨界乾燥法でゲル原料を乾燥させ、図１８（ｃ）に示すように、乾燥ゲルからなる音響整合層３を下層の音響整合層上９に形成する。
その後、図１８（ｄ）に示すように、圧電体４が固定された状態の構造支持体６に音響整合層を接着する。なお、構造支持体６を用いず、図１８（ｃ）の音響整合層を圧電体４に直接接着しても良い。
なお、接着時に加圧によって乾燥ゲルが破壊されないように、最適な加圧条件を選択することが好ましい。乾燥ゲルの圧縮方向の応力に対する強度は比較的高いため、上記接着工程で製造歩留まりが低下することは殆ど無い。
なお、上層の音響整合層３の弾性率と下層の音響整合層９の弾性率とが相互に近い値を示すように、音響整合層３および音響整合層９の材料を選定することが好ましい。両者の弾性率が近いと、接着面の全体に均一な圧力を与えることができ、感度に高い超音波送受信器を高い歩留まりで製造することが容易になる。
図１８（ａ）から（ｄ）に示す方法では、音響整合層９の上に乾燥ゲルを形成する工程において、圧電体や構造支持体を取り扱う必要が無いため、乾燥装置などの設備が小型で済み、低コストで超音波送受波器を製造することが可能となる。
乾燥ゲルの形成工程においては、接着層などの有機物に化学的な負荷がかかる可能性があるが、接着工程を乾燥ゲル形成の後に行うことにより、接着部分を劣化させない。
（実施形態９）
図１９を参照しながら、本発明の他の実施形態を説明する。
本実施形態に特徴的な点は、保護部２が音響整合層３が形成された領域の外周部だけでなく、当該領域の内部にも形成されていることにある。
ゲル原料から湿潤ゲルを経て乾燥ゲル層を形成する際、乾燥ゲル層の上面に凹凸が形成される場合がある。また、湿潤ゲルの乾燥を超臨界乾燥によらずに通常の乾燥法で行う場合、乾燥ゲルの収縮が起こるため、図４に示すような凹部が乾燥ゲルに形成されやすい。
超音波送受波面が広い場合、上記の凹凸は大きくなりやすく、音響整合層の厚さを最適値に設定していても、現実には場所によっては音響整合層の厚さが最適な値から大きくシフトしてしまうことになる。
乾燥ゲル層中の音速は著しく遅いため、音響整合層として適切に機能させるには、極めて薄く形成する必要があり、厚さの許容される誤差範囲も小さい。
図１９（ａ）または図１９（ｂ）に示すようなレイアウトを有する保護部を形成すれば、音響整合層の厚さの誤差を目標値から±５％以内程度に抑えることができる。
図１９（ａ）または図１９（ｂ）に示すように、超音波送受信器における超音波放射面の内部にも保護部２を設けると、音響整合層３の厚さの変動を最小に抑えることができるが、超音波放射面の内側に設けられた保護部２は、超音波の送受信に対して阻害要因となり得る。保護部２による音響的な特性劣化を防止するために、保護部２の大きさを、保護部２としての役割を果たす範囲においてできる限り小さくすることが好ましい。
図１９（ａ）の構成例では、断面が円形の保護部２をランダムに配置しているが、保護部２の断面形状は円形に限定されず、矩形や多角形であってもよい。また、その配列もやランダム配置に限定されない。
図１９（ｂ）の構成例では、同心円状の保護部２が設けられている。この構成例では、超音波送受信器における超音波放射面の内部にも保護部２が存在しているが、超音波送受信器の特性劣化が防止される。図１９（ｂ）の構成は、超音波送受器の中心軸上において超音波放射面から近距離Ｌだけ離れた位置へ超音波を送信する場合に有効である。
保護部２と超音波放射面の中心との間の距離をｒすると、距離ｒは、次の式（６）を満足することが好ましい。

ここで、λは超音波が伝搬する気体中の波長であり、Ｌは超音波送受信器の超音波放射面からの距離である。例えば、周波数が５００ｋＨｚ、超音波伝搬媒体が空気（音速３４０ｍ／ｓ）、測定距離Ｌが１０ｍｍの場合、（式６）から、中心からの半径ｒが２．６〜３．７ｍｍ、４．６〜５．４ｍｍ、６．１〜６．７ｍｍ・・・の位置に保護部２を設けることが好ましいことがわかる。このような位置に保護部２を設けると、音の干渉による音場の乱れを防止し、近距離における超音波の感度劣化を防止するのに有効である。
超音波送受信器の音波放射面の各点を、それぞれ、点音源に見立てると、それぞれの点音源から放射された球面波を合成したものが送信される超音波となる。超音波放射面からの距離が短い場合、位相の異なる超音波が相殺するため、高出力の超音波を送信することができない位置が存在する。超音波放射面から位相の同じ超音波のみを放射させるためには、位相の異なる超音波を放射する領域に保護部２を設けることが有効である。このような領域に保護部２が設けられると、位相の異なる超音波の放射を抑えることができるため、近距離における音場の乱れを防止して、高出力の超音波送信が可能となる。
（実施形態１０）
図２０は、本発明による超音波送受波器の第１０の実施形態を示す断面図である。本実施形態の超音波送受波器２１は、圧電体２２と、圧電体２２の両面に設けられた電極２３ａ、２３ｂと、圧電体２２上に電極２３ａを介して設けられた保護整合層（第１音響整合部）４と、圧電体２２上に電極２３ａを介して設けられた音響整合層（第２音響整合部）２５とを備えている。
図２１は、図２０に示した超音波送受波器２１の上面図である。図２１からわかるように、本実施形態の超音波送受波器は、厚さ（高さ）の異なる保護整合層２４と音響整合層２５とが交互に同心円状に配置された構造を有している。
本実施形態における圧電体２２は、圧電性を有する材料から形成され、厚さ方向に分極されている。圧電体２２の上下面に設けられた電極２３ａ、２３ｂに電圧が印加されると、電圧信号に基づいて圧電体２２で超音波が発生し、保護整合層２４および音響整合層２５を介して超音波伝搬媒体（気体など）２６へ放射される。また、超音波伝搬媒体２６を伝播してきた超音波は、保護整合層２４および音響整合層２５を介して圧電体２２へ伝播する。入射してきた超音波によって圧電体２２は変形し、電極２３ａと電極２３ｂとの間に電圧信号が発生する。
圧電体２２の材料は任意であり、種々の公知材料から形成したものを用いることができる。圧電体２２の代わりに公知の電歪体を用いてもよい。電極２３ａ、２３ｂは好ましくは金属から形成されるが、金属以外の導電材料から形成されていても良い。
保護整合層２４および音響整合層２５は、圧電体２２で発生した超音波振動を伝搬媒体２６へ効率よく伝搬させ、また、超音波伝搬媒体２６を伝搬してきた超音波を効率よく圧電体２２へ伝える機能を有している。
本実施形態の音響整合層２５は、好ましくは、乾燥ゲルから形成される。乾燥ゲルは、ゾルゲル反応によって形成される多孔質体であり、密度ρと音速Ｃとの積（ρ×Ｃ）で規定される音響インピーダンスを極めて小さくすることが可能な材料である。このため、乾燥ゲルから形成した音響整合層２５を用いることにより、空気などの気体に対する超音波の送受波効率を極めて高くすることができる。
乾燥ゲルは、湿潤ゲルを形成した後、この湿潤ゲルを乾燥することによって得られる。湿潤ゲルは、まず、ゲル原料液を用意し、このゲル原料液の反応によって湿潤ゲルを作製することができる。湿潤ゲルは、ゲル原料液の反応によって固体化した固体骨格部を有しており、この固体骨格部が溶媒を含んだ状態にある。
湿潤ゲルを乾燥することによって得られる乾燥ゲルは、多孔質体であり、数ｎｍ〜数μｍ程度の固体骨格部の隙間に連続した気孔を有している。気孔の平均サイズは１ｎｍ〜数μｍ程度と極めて小さい。
作製条件を調節して乾燥ゲルの密度を小さくしてゆくと、乾燥ゲルの固体部分における音速が極端に小さくなるとともに、細孔内の気体部分における音速も極端に小さくなる。そのため、乾燥ゲルの音速は、低密度状態で５００ｍ／秒以下の低い値を示し、極めて低い音響インピーダンスを示すことになる。特に固体骨格部および細孔径が数ｎｍ程度と小さいサイズを持つ乾燥ゲルは極めて低い音速を示す。また、ナノメートルサイズの細孔部では気体の圧損が大きいため、乾燥ゲルから音響整合層を形成した場合、音波を高い音圧で放射できる。
後述する製造方法によれば、同じ原料を用いても製造プロセス条件を調節することにより、乾燥ゲルの音響インピーダンスを広い範囲内で任意に値に制御することができる。また、製造プロセス条件を変えることにより、密度が略同程度の大きさでありながら、音速だけを変化させた音響整合層を作製することも可能である。
乾燥ゲルは、このような有利な特徴を有するが、機械的強度が低い。このため、製造歩留まりを高くすることが困難であり、使用時における信頼性も低かった。このように機械的強度の低い乾燥ゲルを保護する部材を設けることにより、製造歩留まりおよび信頼性が向上することは、実施形態１〜９について示したとおりである。
実施形態１〜９における保護部は、超音波送受波器の製造歩留まり、あるいは使用時における信頼性を向上させるのに極めて有効であり、さらに音響整合層の厚さを高精度に制御しうるため、超音波送受波器の性能安定化に対して有効である。しかし、前述のように、圧電体が超音波を放射または受け取る面（主面）上に保護部を設けると、その保護部が音響的な障害となり得る。これは、また、実施形態１〜９では、音響整合層の材料とは異なる材料から形成される上記保護部の厚さが、音響整合層の厚さと略等しくなるように設定されているため、保護部と音響整合層との間で音速が異なり、音響整合層と略同じ厚さの保護部は音響整合層の役割をしないためである。このため、実施形態１〜９の保護部は、超音波の送受信に対して阻害する要因となり得るため、圧電体の主面の外側に配置することが好ましい。
しかし、更に厳しい環境条件に対する信頼性の確保や、超音波送受波器の外径の制限などによっては、圧電体上部に保護部を設けざるを得ない場合がある。
本実施形態では、圧電体の主面に、音響整合層２５を保護する機能を果たす保護部（密度が相対的に高く、音響整合層２５よりも機械的強度が高い材料から形成される）を有しながらも、超音波送受波器としての性能を損なわない構成を採用する。
本実施形態では、圧電体２２の主面に設けられた保護部の厚さを送受信する超音波の波長の約１／４に設定している。これにより、機械的強度が相対的に高い保護部も音響整合層として機能する。このため、本明細書では、このような保護部を「保護整合層」と称する場合がある。このような構成を採用することにより、音響整合層を保護する保護部も音響整合層としての役割を果たすため、高感度な超音波送受波器を実現することができる。
音響整合層としての機能を最もよく発揮する厚さは、超音波の波長の１／４である。一方、保護整合層２４における音速と音響整合層２５における音速は異なる。このため、保護整合層２４の厚さＬ３と音響整合層２５の厚さＬ１とは、図２０に示されるように、異なる大きさを有している（Ｌ３＞Ｌ１）。
保護整合層２４および音響整合層２５の厚さがいずれも音速の１／４程度に設定されると、保護整合層２４の厚さが音響整合層２５の厚さと異なるため、音響整合層２５の上面から放射された超音波と、保護整合層２４の上面から放射された超音波が干渉する場合がある。高感度な超音波送受波器を実現するためには、それぞれから放射される超音波の位相関係が極めて重要となる。
図２２（ａ）は、保護整合層２４の上面における超音波の波形を示し、図２２（ｂ）は、音響整合層２５の上方において、保護整合層２４の上面と同じレベルにおける超音波の波形を示している。なお、図２２（ｂ）における符号「ｔａ」は、超音波が超音波伝搬媒体２６を伝搬する時間を示している。各グラフにおける横軸の１目盛りは、超音波の周波数が５００ｋＨｚのとき、約３μ秒である。
音響整合層２５の上面から放射された超音波は、気体などの超音波伝播媒体２６を通って保護整合層２４の上面と同じレベルに達する。このため、伝搬媒体２６における音速や伝播媒体２６のサイズＬ２によっては、音響整合層２５の上方において、保護整合層２４の上面と同じレベルにおける超音波の波形の位相関係が変化する。
なお、図２２（ａ）および（ｂ）の信号波形は、保護整合層２４および音響整合層２５から放射される超音波の波長および振幅が等しいと仮定して求めたものである。
保護整合層２４の厚さＬ３および音響整合層２５の厚さＬ１が、それぞれ、各層における超音波波長の１／４であるとき、保護整合層２４の下面と上面との間を超音波が伝搬するに要する時間は、音響整合層２５の下面と上面との間を超音波が伝搬するに要する時間に等しい。従って、音響整合層２５の上面から放射された超音波が保護整合層２４の上面と同じレベルの位置に達した超音波の位相は、保護整合層２４を伝搬して保護整合層２４の上面に達した超音波の位相に比べて、遅れている。この位相の遅れは、音響整合層２５の上面から出た超音波が伝搬媒体２６の中を距離Ｌ２だけ伝播する時間に対応している。
送受信する超音波の周波数をｆ［秒^−１］とすると、超音波の１波長に等しい距離だけ超音波が進むのに必要な時間は１／ｆ［秒］である。超音波が本実施形態の保護整合層２４を通過するのに必要な時間ｔ３は、１／４ｆ［秒］である。一方、超音波が本実施形態の音響整合層２５を通過するのに必要な時間ｔ２も、１／４ｆ［秒］である。ここで、超音波が伝搬媒体２６の中をＬ２の距離だけ伝搬するために必要な時間をｔ２（＝ｔａ）とすると、時間ｔ２に依存して、保護整合層２４の上面から放射された超音波と音響整合層２５の上面から放射された超音波との間に干渉が発生する。この干渉により、超音波の波形および感度が変化する。
図２２（ｃ）は、時間ｔ２が１／２ｆ［秒］である場合に観測される超音波波形を示しており、図２２（ｄ）は、時間ｔ２が１／ｆ［秒］である場合に観測される超音波波形を示している。図２２（ｃ）および（ｄ）からわかるように、時間ｔ２の値により、観測される超音波の感度が大きく異なる。時間ｔ２が１／２ｆ［秒］と等しいとき、位相のずれが超音波の半波長となり、観測される超音波の感度は低くなる。一方、時間ｔ２が１／ｆ［秒］のとき、位相のずれが超音波振動子の波長の整数倍となるため、観測される超音波の感度は高くなる。時間ｔ２が１／２ｆ〜１／ｆ［秒］の範囲内にあるとき、ｔ２が１／２ｆ［秒］から１／ｆ［秒］に近づくほど、超音波の送受信感度が上昇する。
音響整合層２５から放射された超音波が伝搬媒体２６を伝搬して保護整合層２４の上面と同じレベルに達したとき、その超音波の位相が保護整合層２４を伝搬してきた超音波の位相と略一致するように音響整合層２５および保護整合層２４の厚さを調節すると、高感度の超音波送受波器を提供できる。なお、本明細書で「位相が略一致する」とき、超音波の位相の差が超音波波長の１／４以下になることを意味し、位相の差は小さいほど好ましい。
図２３は、時間ｔ２が１／ｆ［秒］である場合の超音波の位相を模式的に示した断面図である。この図では、保護整合層２４の上面における超音波の位相と、音響整合層２５の上方であって保護整合層の上面と同レベルにおける超音波の位相とが一致している。このような位相の一致が生じしたとき、超音波の送受信感度が最大化される。なお、このような位相の完全な一致が生じない場合でも、位相のずれが少なく設定されると、超音波の送受信感度は従来よりも充分に向上される。位相のずれは、超音波伝搬媒体における超音波波長の１／４以下に調節されていることが好ましく、１／８以下に調節されていることが更に好ましい。
音響整合層２５の厚さＬ１および保護整合層２４の厚さＬ３を、それぞれ、音響整合層２５および保護整合層２４における超音波波長の１／４程度に制御するだけでは、Ｌ２の大きさは（Ｌ３−Ｌ１）として一義的に決まるため、ｔ２を任意に設定することができない。このため、時間ｔ２が所望の大きさになるようにするには、音響整合層２５や保護整合層２４の厚さだけではなく、音響整合層２５や保護整合層２４における音速を適切に制御する必要がある。本発明の好ましい実施形態では、音響整合層２５を音速の制御が容易な乾燥ゲルから形成する。
次に、図２４（ａ）から（ｃ）を参照しながら、本実施形態の超音波送受波器２１の製造方法の実施形態を説明する。本実施形態においては、超音波伝搬媒体２６として空気（密度：１．１８ｋｇ／ｍ^３、音速：約３４０ｍ／ｓ、音響インピーダンス約４．０×１０^２ｋｇ／ｍ^２／ｓ）を考える。
まず、図２４（ａ）に示すように、送受信する超音波の波長に合わせた圧電体２２を用意する。この段階の圧電体２２には、図２４（ａ）に示す保護整合層２４は設けられていない。圧電体２２としては、圧電セラミックスや圧電単結晶など圧電性の高い材料が好ましい。圧電セラミックスとしては、チタン酸ジルコン酸鉛、チタン酸バリウム、チタン酸鉛、ニオブ酸鉛などを用いることができる。圧電単結晶としては、チタン酸ジルコン酸鉛単結晶、ニオブ酸リチウム、水晶などを用いることができる。
本実施形態では、圧電体２２としてチタン酸ジルコン酸鉛セラミックスを用い、送受信する超音波の波長を５００ｋＨｚに設定している。このような超音波を圧電体２２が効率よく送受信できるようにするため、圧電体２２の共振周波数を５００ｋＨｚに設計する。このため本実施形態では、直径が１２ｍｍ、厚さが約３．８ｍｍの円柱形状を有する圧電セラミックスから形成された圧電体２２を用いている。圧電体２２の両面には銀の焼付けによる電極２３ａ、２３ｂが形成され、この方向に分極処理が施されている。
次に、保護整合層２４としての機能する３つのリング状部材を用意し、図２４（ａ）に示すように圧電体２２の主面に接合する。このとき、図２１に示すように、リング状部材の各中心が圧電体２２の中心に揃うようにする。保護整合層２４としての機能する３つのリング状部材は、それぞれ、外径１２ｍｍ、内径１１ｍｍ、厚さ１．０ｍｍの第１リング状部材、外形８ｍｍ、内径７ｍｍ、厚さ１．０ｍｍの第２リング状部材、および、外形４ｍｍ、内径３ｍｍ、厚さ１．０ｍｍの第３リング状部材である。
本実施形態における保護整合層２４には、機械的強度が高く、音響整合層を保護できる機能が求められるだけでなく、音響整合層の機能を果たすために比較的低い音響インピーダンスを有することが求められる。このような材料として、本実施形態では、多孔質体のセラミックスを用いる。この多孔質セラミックスは、見かけ密度が０．６４×１０^３ｋｇ／ｍ^３、音速が２０００ｍ／ｓ、音響インピーダンスが約１．２８×１０^６ｋｇ／ｍ^３である。セラミックスとしては、チタン酸バリウム系の材料を用いている。なお、「見かけ密度」とは、多孔質体に含まれる空間部分をも含んだ密度である。多孔質セラミックスは、体積の約８０％程度が空間部分であり、セラミックスの実体部分は約２０体積％である。
上述のように、保護整合層２４の音速が約２０００ｍ／ｓであるため、５００ｋＨｚにおける波長の１／４の厚さは１．０ｍｍに相当する。このため、本実施形態では保護整合層２４として機能するリング状部材の厚さを１．０ｍｍに設定している。
本実施形態で用いる多孔質セラミックスは、次のようにして作製され得る。
まず、樹脂製の微小なボールとセラミックス粉末を混合、加圧成形する。その後、セラミックスを焼結する。この焼結過程において、樹脂ボールは加熱され、燃焼して除去される。焼結に際して、加熱を急激に行うと、樹脂ボールが膨張または急激なガス化を起こし、セラミックス構造体を破壊してしまうおそれがある。このため、焼結は緩やかな加熱によって行うことが好ましい。
本実施形態では、このような多孔質セラミックスから形成したの保護整合層２４と圧電体２２とを接着剤による接着によって接合する。例えば、接着剤としてはエポキシ系樹脂を用い、０．１ＭＰａ程度の圧力をかけながら、１５０℃の恒温槽中で２時間程度放置すると、接着剤は硬化し、保護整合層２４と圧電体２２とが接合する。
次に、こうして形成した圧電体２２／保護整合層２４からなる複合体上に、図２４（ｂ）に示すように音響整合層２５を設ける。本実施形態では、音響整合層２５を乾燥ゲルから形成する。
本実施形態では、まず、図２４（ｂ）に示す厚さの音響整合層２５を形成した後、図２４（ｃ）に示すようように音響整合層２５を薄くする。このとき、図２０に示す距離Ｌ２（＝Ｌ３−Ｌ１）が空気中に超音波の１波長に等しくなるように、保護整合層２４の厚さＬ３および音響整合層の厚さＬ１を設定する。具体的には、送受信する超音波の周波数が５００ｋＨであるので、この超音波の空気中における１波長は、０．６２ｍｍである。一方、保護整合層２４の厚さＬ３は１．０ｍｍであるため、音響整合層２５の厚さＬ１は、０．３２ｍｍ（＝１．０ｍｍ−０．６２ｍｍ）とになる。また、音響整合層２５が音響整合層として適切に機能するためには、この厚さＬ１（＝０．３２ｍｍ）が音響整合層２５を伝搬する超音波の波長の１／４となることが最も望ましい。従って、０．３２ｍｍが送受信する超音波の波長の１／４となるような音速を有する材料特性を有することが必要となる。計算によれば、音速が６４０ｍ／ｓとなるような乾燥ゲルから厚さ０．３２ｍｍの音響整合層２５を形成すれば良い。
なお、保護整合層２４の厚さは、保護整合層２４における超音波波長の１／４であることが好ましいが、その大きさに限定されるわけではない。超音波波長の１／８以上１／３以下の範囲であれば良く、超音波波長の１／６以上１／４以下の範囲であることが更に好ましい。超音波の波長に分布がある場合、ピーク波長を基準にして厚さを決定することが好ましい。本明細書においては、波長に分布からある場合、「波長の１／４」とは「ピーク波長の１／４」を意味するものとする。
音響整合層２５が単層である場合、音響整合層２５の厚さも、音響整合層２５における超音波波長の１／４であることが好ましいが、その大きさに限定されるわけではない。超音波波長の１／８以上１／３以下の範囲であれば良く、超音波波長の１／６以上１／４以下の範囲であることが更に好ましい。音響整合層が多層構造を有している場合、各構成層が上記の厚さを有していることが好ましい。音響整合層が多層構造を有している超音波送受波器は、実施形態２として後述する。
音響整合層２５を構成する乾燥ゲルの材質としては、無機材料、有機高分子材料など様々な材料を用いることができる。無機材料の固体骨格部としては、酸化ケイ素（シリカ）、酸化アルミニウム（アルミナ）、酸化チタンなどを用いることができる。また有機材料の固体骨格部としては、一般的な熱硬化性樹脂、熱可塑性樹脂を用いることができ、例えば、ポリウレタン、ポリウレア、フェノール樹脂、ポリアクリルアミド、ポリメタクリル酸メチルなどを用いることができる。
本実施形態では、音響整合層２５の材料として、コスト、環境安定性、製造のしやすさ、超音波送受波器の安定な温度特性などの観点から、固体骨格部として酸化ケイ素（シリカ）を持つ乾燥ゲルを採用する。
６４０ｍ／ｓの音速は、乾燥ゲルの音速としては比較的高い値である。このため、本実施形態では、音響整合層２５として乾燥ゲル層を形成する際に、ゲル化工程（以下、「第１ゲル化工程」と称する。）に引き続いて乾燥工程を行う従来の製造方法ではなく、第１ゲル化工程後に第２ゲル化工程を行う方法を採用する。
第２ゲル化工程を行わずに第１ゲル化工程だけを行う場合は、相対的に高い音速を示す乾燥ゲルを得ることが困難である。なお、乾燥ゲルの密度は、音速と略比例して高くなるため、「高い音速」は「高い密度」を意味する。ゲルの音速を高くする目的で、ゲル原料液中におけるゲル原料の濃度を高くすると、ゲル化反応が均一に進行せず、ランダムな音速分布を持つ湿潤ゲルが形成される。この湿潤ゲルを乾燥することによって得られる乾燥ゲルも、ランダムな密度分布を持つこととなる。このため、ゲル原料液中におけるゲル原料の濃度を高くすると、音速を均一化することは極めて難しくなる。
本実施形態では、ゲルの不均一化を避けるため、第１ゲル化工程で形成する乾燥ゲルの音速は２００ｍ／ｓ程度以下に調節し、第２ゲル化工程によって密度を更に上昇させ、均一に音速を上昇させる。第２ゲル化工程では、第１ゲル化工程で得られた湿潤ゲルを再びゲル原料液（第２ゲル化原料液）に浸漬する処理を行う。そして、第２ゲル化工程では、第２ゲル化原料液中の触媒となるアンモニアの濃度を低く調整する。このため、第１ゲル化工程で得られた湿潤ゲルの外ではゲル化が起こらない。しかし、第１ゲル化工程で得られた湿潤ゲルの内部では、第１ゲル化工程で形成された骨格に第２ゲル化原料液が付着していくように成長する。このため、ゲル原料液自体がゲル化しない条件においても、この反応は進行する。このようにしてゲルの音速、密度を変化させることが可能となる。
具体的には、以下に示す工程を行うことにより、乾燥ゲルによる音響整合層２５を形成する。
工程１： 第１ゲル化ゲル原料液の用意
テトラエトキシシラン／エタノール／水／塩化水素を、モル比で１／２／１／０．０００７８で混合して、６５度の恒温槽中で３時間、テトラエトキシシランの加水分解を進行させる。更に、水／ＮＨ_３を、２．５／０．００５７の割合（テトラエトキシシランに対するモル比）を加えて混合したゲル原料液を用意する。
工程２： 第１ゲル化工程
上記のようにして調整したゲル原料液（第１ゲル化原料液）を、圧電体２２と保護整合層２４で形成された空間に滴下する。この際、一番外側の保護整合層２４の外周にテフロン製のシートを巻きつけ、ゲル原料液がこぼれないように枠を形成する。
ゲル原料液を滴下したサンプルを恒温槽中で水平を保ちながら５０℃で約１日放置する。こうして、圧電体２２と保護整合層２４とによって形成された空間内に供給されたゲル原料液がゲル化し、湿潤ゲルを形成する。
工程３： 第２ゲル化工程（音速、密度の調整）
第１ゲル化工程で得られた音響整合層に対して、そのまま乾燥工程を行った場合、密度は２．０×１０^２ｋｇ／ｍ^３程度、音速は２００ｍ／ｓ程度となる。本実施形態では、音速、密度を更に高くする目的で第２ゲル化工程を行う。
まず、第１ゲル化工程で得られた湿潤ゲルをエタノールで洗浄し、第２ゲル化原料液を準備する。第２ゲル化原料液として、テトラエトキシシラン／エタノール／０．１規定アンモニア水を、体積比で６０／３５／５を混合したものを用いる。
第１ゲル化工程で得られた圧電体２２／湿潤ゲル／保護整合層２４からなる複合体を密閉容器中の第２ゲル化原料液に浸漬し、７０℃の恒温槽中で約４８時間放置する。この第２ゲル化工程により第１ゲル化工程で得られたゲル骨格が成長し、密度、音速が高くなる。
工程４： 疎水化工程
疎水化工程は、必ずしも必要ではないが、吸湿により性能が劣化することがあるため、行うことが好ましい。疎水化工程は、第２ゲル化工程の後、湿潤ゲル内に残留している、第２ゲル化原料液をエタノールにより置換・洗浄した後、ジメチルジメトキシシラン／エタノール／１０重量％アンモニア水を、重量比で４５／４５／１０の割合で混合して得られた疎水化液に、４０℃で、約１日間、浸漬することによって、疎水化工程を行う。
工程５： 乾燥工程
以上の工程で得られた湿潤ゲルから、乾燥ゲルを得るために、乾燥工程を行う。本実施形態では、乾燥方法として、超臨界乾燥法を用いる。乾燥ゲルは前述のように、非常に小さなナノメートルサイズ程度の多孔質体であり、骨格部分の太さや、結合の強さ、空孔の大きさによっては、湿潤ゲルから乾燥ゲルへの溶媒乾燥の際に、溶媒の表面張力によって、破壊されてしまうことがある。
このため、表面張力の働かない超臨界乾燥法が有用に利用することができる。具体的には、上述の疎水化液をエタノールで置換した後、以上の工程で得られた圧電体２２／湿潤ゲル／保護部研音響整合層２５の複合体を耐圧容器に入れて、湿潤ゲル内のエタノールを液化二酸化炭素に置換する。
更に容器内にポンプで液化二酸化炭素を送り込むことにより、容器内の圧力を１０ＭＰａまで上昇させる。その後、５０℃まで昇温することで容器内を超臨界状態とした。次に温度を５０℃に保ったまま、圧力をゆっくり開放することで乾燥を完了する。
工程６： 厚さ調整工程
こうして形成した乾燥ゲル層を、旋盤によりその厚さを０．３２ｍｍとなるように音響整合層２５のみの部分を研削した。
このようにして得られた音響整合層２５を形成する乾燥ゲルの密度は、約０．６×１０^３ｋｇ／ｍ^３であり、音速は約６４０ｍ／ｓとなる。また保護整合層２４の一部に、音響整合層２５となる乾燥ゲルが浸透しているが、保護整合層２４の音速には影響を与えない。
乾燥ゲルから音響整合層を形成する工程の前に、電極２３ｂと音響整合層２５との密着性が良くなるように電極２３ｂの表面を処理するすることが好ましい。表面処理によって電極２３ｂと音響整合層２５との密着性が増すと、信頼性が更に向上する。このような表面処理としては、乾燥ゲルと化学的な結合をしやすい圧電体表面の電極に水酸基が付与されるようなプラズマ処理などを採用することができる。あるいは、電極２３ｂの表面に物理的な凹凸を形成することによってアンカー効果を付与することも有効である。具体的には、化学的および／または物理的なエッチング処理を好適に採用することができる。
本実施形態では、音響整合層２５となるゲルを形成した後、旋盤による研削を行い、乾燥ゲルの厚さを調整する。厚さの調節は、第１ゲル化工程の際に滴下する第１ゲル化原料液の量（高さ）を調整することによって行ってもよい。この場合には、形成される音響整合層の厚さが最終的に０．３２ｍｍ程度となるように、３３．９μＬのゲル原料液をマイクロピペットで正確に量り取り、圧電体２２上に滴下する。保護整合層２４が８０％の空隙を有する多孔質体であるため、多孔質体に吸収される体積を換算して滴下量を計算する必要がある。
このようにして製造した超音波送受波器の送受信波形を図２５に示す。図２５において、本実施形態の超音波送受波形は実線で示され、保護部と音響整合層２５とが同じ厚さを有する超音波送受波器（比較例）の超音波送受信波形は点線で示されている。図２５からわかるように、本実施形態によれば、信号の振幅が増加する。
本発明の構造を用いることで高感度化を達成できる。
本実施形態では、保護整合層２４の上面と同一レベルの位置における超音波の位相を揃えるため、音響整合層２５および伝搬媒体２６を伝搬してきた超音波が、保護整合層２４を伝搬してきた超音波に比べて、ちょうど波長分だけ位相の遅れを生じるようにしている。さらに大きな音速を有する材料から保護整合層２４を形成する場合や、保護整合層２４の厚さＬ３を大きく設定する場合には、伝搬媒体２６による位相遅れを超音波波長の２波長以上に設定してもよい。
（実施形態１１）
次に、図２６を参照しながら、本発明の超音波送受波器の第１１の実施形態を説明する。本実施形態の主な特徴点は、音響整合層が下層の第１音響整合層２５ａおよび上層の第２音響整合層２５ｂを含む積層構造を有している点である。
音響整合層２５が２層構造を有する場合にも、各音響整合層２５ａ、２５ｂの厚さのそれぞれを、各音響整合層における超音波波長の１／４程度に設定することが好ましい。
本実施形態でも、第１０の実施形態と同様に、保護整合層２４の上面と同一レベルの位置において超音波の位相を揃えるため、音響整合層２５および伝搬媒体２６を伝搬してきた超音波が、保護整合層２４を伝搬してきた超音波に比べて、超音波波長の略整数倍分だけの位相の遅れを生じるようにしている。
図２６に示す構成では、超音波が保護整合層２４を伝播して保護整合層２４の上面に達したとき、その超音波と同位相の超音波は第１音響整合層と第２音響整合層２５ｂの境界面に達している。これは、第１音響整合層２５ａの音速が保護整合層２４における音速よりも小さいためである。超音波が第１音響整合層２５ａの上面から第２音響整合層２５ｂの上面に達するまでに、更に１／４ｆ［秒］の時間がかかる。このため音響整合層２５ｂの上面から伝搬媒体２６を伝搬して保護整合層２４の上面と同じレベルに達するまでの時間が３／４ｆ［秒］となるように設定すると、保護整合層２４の上面レベルで位相が揃う。このような構成を採用すると、音響整合層２５ａおよび２５ｂを透過して放射された超音波と、保護整合層２４を透過して放射された超音波との間に、１波長分のずれが生じ、両超音波が干渉して強め合うため、超音波の振幅が大きくなる。
本実施形態における音響整合層２５ａ、２５ｂの製造方法を説明する。
まず、第１０の実施形態における音響整合層２５の製造方法と同様にして、保護整合層２４を作製する。保護整合層２４の材料として多孔質セラミックスを用い、その厚さ（Ｌ７）を１．０ｍｍに設定する。
本実施形態では、空気などの伝搬媒体２６を伝搬する時間が３／４ｆ［秒］となるように、第２音響整合層２５ｂの上面から保護整合層２４の上面レベルまでの距離（Ｌ６）を０．５１ｍｍに設定する。この結果、第１音響整合層２５ａと第２音響整合層２５ｂの合計厚さ（Ｌ４＋Ｌ５）は、０．４９ｍｍに等しくなる。
本実施形態では、第２音響整合層２５ｂの音速を２００ｍ／ｓに設定すると、第２音響整合層２５ｂの厚さ（Ｌ５）は０．１０ｍｍに設定することが好ましい。Ｌ５＝０．１０ｍｍとすると、第１音響整合層２５ａの厚さ（Ｌ４）は０．３９ｍｍ（＝０．４９ｍｍ−０．１０ｍｍ）となる。第１音響整合層２５ａの厚さが第１音響整合層２５ａにおける超音波の１／４波長に相当するようにするには、第１音響整合層２５ａの音速を７８０ｍ／ｓにする必要がある。
次に、上記のような２層の音響整合層２５ａ、２５ｂの作製方法を説明する。この方法で特徴的な点は、第１０の実施形態で行った第２ゲル化工程を２度行うことにある。すなわち、本実施形態では、第１ゲル化工程で形成された湿潤ゲルの外側ではゲル化をしない第２ゲル化工程（第２−１ゲル化工程）を行った後に、湿潤ゲルの外側でもゲル化が生じる第２ゲル化工程（第２−２ゲル化工程）を行う。
本実施形態では、まず、第１０の実施形態で行った工程１〜工程６と同様の工程を行うことにより、第１音響整合層２５ａを形成する。ただし、このときの第２ゲル化工程は、第２−１ゲル化工程である。
この後に行う第２−２ゲル化工程における音響インピーダンスの増加を見込み、第２−１ゲル化工程では、第１音響整合層２５ａの密度約０．５×１０^３ｋｇ／ｍ^３、音速５００ｍ／ｓ程度となるように処理時間を調節する。本実施形態では、処理時間を第１０の実施形態における第２ゲル化工程の処理時間よりも短縮し、約３６時間に設定する。
次に、第２−２ゲル化工程を行うことにより、第１音響整合層２５ａの音響インピーダンスを増加ざせるとともに、第１音響整合層２５ａの上部に第２音響整合層２５ｂを形成する。この第２−２ゲル化工程は、具体的には、以下のようにして行った。
第２−２ゲル化工程：
まず、第２−２ゲル化原料液として、テトラエトキシシラン／エタノール／０．０５規定アンモニア水をモル比で、１／４／３の割合で混合した液を用意する。この第２−２ゲル化原料液を、第１音響整合層２５ａと保護整合層２４によって形成された空間内に充填する。次に、このまま室温で約２４時間放置することにより、ゲル化を完了する。こうして、第１音響整合層２５ａの音響インピーダンスを調整するとともに、第２音響整合層２５ｂとなる湿潤ゲルを形成する。
この後、第１０の実施形態と同様にして、疎水化工程、乾燥工程、および厚さ調整工程を行うことにより、音響整合層２５ａ、２５ｂを完成する。本実施形態における音響整合層２５ａ、２５ｂは、以下のように特徴付けられる。
第１音響整合層２５ａ
密度：０．７×１０^３ｋｇ／ｍ^３、音速：７８０ｍ／ｓ
音響インピーダンス：５．４６×１０^５ｋｇ／ｍ^２／ｓ
厚さ：０．３９ｍｍ
第２音響整合層２５ｂ
密度：０．２×１０^３ｋｇ／ｍ^３、音速：２００ｍ／ｓ
音響インピーダンス：４．０×１０^４ｋｇ／ｍ^２／ｓ
厚さ：０．１０ｍｍ
本実施形態の超音波送受信器の送受信波形を図２７に示す。図２７において、本実施形態の超音波送受波器の超音波送受波形は実線で示され、音響整合層と保護整合層の厚さを等しくした超音波送受波器（比較例）の送受波形形は点線で示される。図２７からわかるように、本実施形態の超音波送受波器によれば、高感度化を実現することができる。
本実施形態では、音響整合層２５を２層とした構成としたが、３層以上としても、保護整合層２４の上面部分で、超音波の位相が揃うように設計することで同様の効果が得られる。
（実施形態１２）
図２８を参照しながら、本発明による超音波送受波器の第１２の実施形態を説明する。本実施形態の特徴的な点は、第１音響整合層２５ａと保護整合層２４とが、同じ材料によって一体的に形成されている点にある。第１音響整合層２５ａの上部には乾燥ゲルから形成した第２音響整合層２５ｂが形成されている。
本実施形態では、第１音響整合層２５ａおよび保護整合層２４における超音波の音速や波長が等しく、しかも、保護整合層２４の厚さＬ１１を超音波の１／４波長に設定する。このため、第１音響整合層２５ａの厚さＬ８は超音波の１／４波長よりも小さい。第１音響整合層２５ａの厚さＬ８は、第２音響整合層２５ｂの厚さＬ９と、第２音響整合層２５ｂの上面から保護整合層２４の上面レベルまでの距離Ｌ１０によって決まる。
本実施形態においても、音響整合層２５ａおよび５ｂを透過して放射された超音波と、保護整合層２４を透過して放射された超音波との間に、超音波波長の整数倍の位相遅れが生じる構成を採用している。このため、保護整合層２４の上面レベルにおいて、音響整合層２５ａ、２５ｂおよび伝搬媒体２６を伝搬してきた超音波の位相が揃う。
音響整合層２５ａ、２５ｂを透過してきた超音波の感度を高めるには、第１音響整合層２５ａの厚さよりも第２音響整合層２５ｂの厚さが重要である。本実施形態では、第２音響整合層２５ｂの厚さは、送受信する超音波の波長の約１／４に設定する。第１音響整合層２５ａの厚さも、感度に影響を与えるが、その影響の多くは周波数の比帯域に及ぶ。
このため、本実施形態では、まず材質の機械的強度などの点から、第２音響整合層２５ｂを構成しうる乾燥ゲル層の特性を決定する。次に、保護整合層２４と同じ材料から形成する第１音響整合層２５ａの厚さＬ８と、音波伝搬媒体の厚さＬ１０とを設定する。
本実施形態では、保護整合層２４の材料として、前述の実施形態と同様に多孔質セラミックスを用い、その厚さ（Ｌ１１）を超音波波長の１／４に設定する。すなわち、Ｌ１１を１．０ｍｍに設定する。この場合、上記の多孔質セラミックスから形成される第１音響整合層２５ｂの音速は２００ｍ／ｓ、密度は０．２×１０^３ｋｇ／ｍ^３となる。乾燥ゲルから形成する第２音響整合層２５ｂの厚さを超音波波長の１／４に設定するため、Ｌ９を０．１０ｍｍとする。
このとき、以下の式６が成立する。

式６は、Ｌ１１を１．０ｍｍ、Ｌ９を０．１ｍｍに設定したことから導かれる。
優れた特性を発揮するには、以下の式が成立することが好ましい。

本実施形態では、周波数が５００ｋＨｚの超音波を送受信するため、音響整合層２５ａにおける超音波の１波長は１．０ｍｍ、音波伝搬媒体２６における超音波の１波長は１７／２５ｍｍとなる。式７は、超音波の１波長に対する第１音響整合層２５ａの厚さＬ８の比率と、超音波の１波長に対する伝搬媒体２６の厚さＬ１０の比率との和である。式８を満足するということは、超音波が第１音響整合層２５ａおよび音波伝搬媒体２６を透過する際に１波長だけ進むことを意味している。言い換えると、超音波が感じする第１音響整合層２５ａおよび音波伝搬媒体２６の実効的な厚さが１波長分であることを意味する。
式６および式７を満足するＬ８およびＬ１０を算出すると、Ｌ８＝約０．６９ｍｍ、Ｌ１０＝０．２１ｍｍとなる。
次に、図２９（ａ）から（ｄ）を参照しながら、本実施形態の超音波送受波器の製造方法を説明する。
まず、図２９（ａ）に示すように、多孔質セラミックスからなる厚さ１．０ｍｍのペレットを用意し、このペレットを図２９（ｂ）に示すように加工する。本実施形態では、ペレットの上面に溝を形成し、溝底部の厚さを０．６９ｍｍに調節する。この溝底部が第１音響整合層２５ａとして機能する部分である。溝は図２１に示すようにリング状に形成する。
次に、図２９（ｃ）に示すように、溝の内部に第２音響整合層２５ｂを形成する。音響整合層２５ｂの厚さが０．１ｍｍになるようにする。保護整合層２４／音響整合層２５ａ、２５ｂの複合体を、図２９（ｄ）に示すように、圧電体２２に接合し、超音波送受波器２１を形成する。
第１音響整合層２５ｂは、テトラメトキシシラン／エタノール／０．０５規定アンモニア水をモル比で、１／７／４の割合で混合した液を用いて、実施形態１と同様にして第１ゲル化工程を行うことによって形成する。
保護整合層２４／音響整合層２５ａ、２５ｂからなる複合体の圧電体への接着は、実施形態１と同様に、エポキシ系の接着剤によって行うことができる。
本実施形態によれば、保護整合層２４と音響整合層２５ａを一括的に形成できるため、製造工程を簡単にし、製造コストを低減できる。
（実施形態１３）
図３０を参照しながら、本発明による超音波送受波器の第１３の実施形態を説明する。本実施形態に特徴的な点は、構造支持体を有している点である。
本実施形態の超音波送受波器は、圧電体２２と第１音響整合層２５や保護整合層２４との間に構造支持体７を有している点を除けば、実施形態１の超音波送受波記録媒体の構成と同様の構成を有している。
構造支持体７は音響整合層２５などが固定される円盤状支持部と、この円盤状支持部から軸方向に連続的に伸びる円筒部とを備えている。円筒部の端面は、断面がＬ字型に折れ曲がり、圧電体２２の遮蔽のためのプレート（不図示）や、他の装置などに固定しやすくなっている。
構造支持体７の表面には音響整合層２５や保護整合層２４が配置されており、支持部裏面には圧電体２２が配置されている。このような構造支持体１３を用いることにより、超音波送受波器の取り扱いが極めて容易となる。
構造支持体は、密閉可能な容器（センサケース）から構成することができる。この場合、構造支持体２７の円筒部の開放端を遮蔽プレートなどで塞ぎ、かつ、構造支持体２７の内部を不活性ガスで満たせば、流量測定の対象とする流体から圧電体２２を遮断することができる。
圧電体２２には電圧が印加されるため、可燃性ガスなどと圧電体が接すると、可燃性ガスに引火する危険性もある。しかし構造支持体２７を密閉性の容器から構成し、圧電体２２のある内部を外部流体などと遮断することによって、そのような引火を防止して、可燃性ガスなどに対しても安全に超音波を送受波することができる。
また可燃性ガスでなくとも、圧電体２２と反応し、圧電体２２に特性の劣化を与える可能性のあるガスとの間で超音波を送受波する場合でも、圧電体２２が外部ガスから遮断することが好ましい。そうすることにより、圧電体２２の劣化を防止し、長期間に渡って信頼性の高い動作を実現することが可能となる。
構造支持体２７のうち、圧電体２２と音響整合層２５や保護整合層２４との間に位置する部分は音響整合層として機能しない。このため、構造支持体２７が音響的な阻害として働かないようにするため、構造支持体２７のうち、圧電体２２と音響整合層２５や保護整合層２４との間に位置する部分の厚さを、送受波する超音波の波長の１／８程度以下とすることが望ましい。
本実施形態では、構造支持体２７をステンレスから形成し、上記部分の厚さを０．２ｍｍに設定している。
ステンレスの音速は約５５００ｍ／秒であり、超音波の５００ｋＨｚにおける波長は約１１ｍｍとなる。０．２ｍｍの厚さは波長の約１／５５に相当するため、構造支持体７の存在は殆ど音響的阻害要因にはならない。
構造支持体２７の材料は、ステンレスなどの金属材料に限定される物ではなく、セラミック、ガラス、樹脂などから目的に応じた材料が選択される。本実施形態では、外部の流体と圧電体を確実に分離し、構造支持体に何らかの機械的な衝撃が加わったとしても、圧電体と外部流体との接触を防止できる強度を与えるため、金属材料から構造支持体２７を作製している。これにより、例えば可燃性や爆発性を有するガスを対象として超音波の送受波を行っても高い安全性を確保することができる。
なお、安全な気体に対して超音波の送受波を行う場合には、コスト低減を目的として、樹脂などの材料からなる構造支持体を用いても良い。
（実施形態１４）
図３１（ａ）および（ｂ）を参照しながら、本発明による超音波送受波器の第１４の実施形態を説明する。図３１（ａ）および（ｂ）は、本実施形態の上面図である。
図２１に示す例では、保護整合層２４として機能する多孔質セラミック製のリング（同じ幅で直径の異なる３つのリング状部材）を用い、それらの中心が一致するように圧電体主面に配置しているが、図３１（ａ）に示すように幅の異なるリングを用いて保護整合層２４を形成しても良い。また、図３１（ｂ）に示すように、島状の保護整合層２４をランダムに配置してもよい。
保護整合層２４と音響整合層２５とが、超音波送受波器の主面上に規則的に配列されている場合、その主面に対して或る角度を持った方向に超音波の位相が揃い、振幅が強まる。これは、「サイドローブ」と呼ばれ、超音波計測を行う上で阻害要因となる。しかし、図３１に示すように、保護整合層２４の配列が周期性を持たない構成を採用することにより、サイドローブを抑制し、精度および信頼性の高い超音波計測を可能とすることができる。
（実施形態１５）
図３２を参照しながら、本発明による超音波送受波器の第１５の実施形態を説明する。
本実施形態の超音波送受波器は、保護整合層２４の厚さが面内分布を有している点に第１の特徴点を有している。前述の各実施形態では、保護整合層２４の厚さが面内で一様に設定されているが、本実施形態では、意図的に面内分布が与えられている。また、本実施形態の第２の特徴点は、圧電体２２上に設けられた保護整合層２４が異なる２種類の材料から形成されていることにある。
本実施形態の構成によれば、異なる材料の採用および／または異なる厚さの面内分布を付与することにより、サイドローブを抑制したり、送受信する超音波の周波数を変化させて広帯域化することができる。
なお、各保護整合層２４の厚さは、超音波波長の１／８以上１／３以下の範囲内に含まれてことが好ましく、超音波波長の１／６以上１／４以下の範囲内に含まれていることが更に好ましい。ただし、異なる厚さを有する保護整合層２４の一部の厚さが上記の範囲から外れていてもよい。上記の範囲から外れた厚さを有する保護整合層２４は、音響整合層としては機能しないため、超音波送受信の感度が低下する。しかし、音響整合層として機能しない保護層（もはや「保護整合層」とは呼べない）を圧電体上の適当な位置に置くことにより、近距離における超音波場の乱れを防止して良好な超音波計測を可能にすることができる。
（実施形態１６）
図３３を参照しながら、本発明による超音波流量計の実施形態を説明する。
本実施形態の超音波流量計は、流量測定部５１として機能する管内を被測定流体が速度Ｖで流れるよう設置される。流量測定部５１の管壁５２には、本発明の超音波送受信器から形成した超音波送信受波器１ａおよび１ｂが相対して配置されている。
ある時点では、超音波送受信器１ａが超音波送波器として機能し、超音波送受信器１ｂを超音波受波器として機能するが、他の時点では、超音波送受信器１ａが超音波受波器として機能し、超音波送受信器１ｂを超音波送波器として機能する。この切り替えは、切替回路５３によって行われる。
超音波送受信器１ａ、１ｂは、切替回路５３を介して、超音波送受信器１ａおよび１ｂを駆動する駆動回路５４と、超音波パルスを検知する受信検知回路５５とに接続されている。受信検知回路５５の出力は、超音波パルスの伝搬時間を計測するタイマ５６に送られる。タイマ５６の出力は、流量を演算する演算部５７に送られる。演算部５７では、測定された超音波パルスの伝搬時間に基づいて、流量測定部５１内を流れる流体の速度Ｖが計算され、流量が求められる。駆動回路５４およびタイマ５６は、制御部５８に接続され、制御部５８から出力された制御信号によって制御される。
以下、この超音波流量計の動作をより詳細に説明する。
被測定流体として、ＬＰガスが流量測定部５１を流れる場合が考える。超音波送信受波器１ａおよび１ｂの駆動周波数を約５００ｋＨｚとする。制御部５８は、駆動回路５４に送信開始信号を出力すると同時に、タイマ５６の時間計測を開始させる。駆動回路５４は、送信開始信号を受けると、超音波送信受波器１ａを駆動し、超音波パルスを送信する。送信された超音波パルスは流量測定部５１内を伝搬して、超音波送信受波器１ｂで受信される。受信された超音波パルスは超音波送信受波器１ｂで電気信号に変換され、受信検知回路５５に出力される。
受信検知回路５５は、受信信号の受信タイミングを決定し、タイマ５６を停止させる。演算部５７は、伝搬時間ｔ１を演算する。
次に、切替回路５３により、駆動回路５４および受信検知回路５５に接続する超音波送信受波器１ａおよび１ｂを切り替える。そして、再び、制御部５９は駆動回路５４に送信開始信号を出力すると同時に、タイマ５６の時間計測を開始させる。伝搬時間ｔ１の測定と逆に、超音波送信受波器１ｂで超音波パルスを送信し、超音渡送信受波器１ａで受信し、演算部５７で伝搬時間ｔ２を演算する。
ここで、超音波送信受波器１ａと超音渡送信受波器１ｂの中心を結ぶ距離をＬ、ＬＰガスの無風状態での音速をＣ、流量測定部５１内での流速をＶ、被測定流体の流れの方向と超音波送信受波器１ａおよび１ｂの中心を結ぶ線との角度をθとする。
伝搬時間ｔ１、ｔ２は、それぞれ、測定によって求められる。距離Ｌは既知であるので、時間ｔ１とｔ２を測定すれば、流速Ｖが求められ、その流速Ｖから流量を決定することができる。
このような超音波流量計において、伝搬時間ｔ１、ｔ２はゼロクロス法と呼ばれる方法によって測定される。この方法では、図２１（ａ）に示すような受信波形に対して、適切なスレッショルドレベルを設定し、そのスレッショルドレベルを超えて次に振幅が０となる点の時間を測定する。受信信号のＳ／Ｎが悪い場合、ノイズレベルによっては振幅が０となる点が時間的に変動するため、正確にｔ１、ｔ２を測定することができず、正確な流量を測定することが困難になる場合がある。
このような超音波流量計の超音波送受波器として、本発明の超音波送受信器を用いると、受信信号のＳ／Ｎが向上し、ｔ１、ｔ２を高い精度で測定することが可能となる。
図２１（ｂ）に示すように、図２１（ａ）の場合に比べて、受信信号の立ち上がりが遅い（狭帯域である）と、スレッショルドレベルの設定値に対して、ｔ１、ｔ２を測定する受信信号の山の位置が変動し、測定誤差となる可能性がある。しかし、本発明による超音波送受信器は広帯域で適切に動作するため、受信信号の立ち上がりがよく、正確な流量測定を安定的に行うことが可能となる。なお、ｔ１、ｔ２の値としては、他数回の測定によって得られた値の平均値を用いることが好ましい。
広帯域の超音波を送受信できるということは、信号の立下りも早いことを意味する。このため、測定の繰り返しを速く行った場合でも、前の送受信信号の影響を受けることが無い。その結果、測定の繰り返し周波数を高くしても、瞬時の計測を可能とするものであり、ガスもれなどを瞬時に検知することが可能となる。
以上の各実施形態では、最上層の音響整合層（第１音響整合層）の上面は露出しているが、この面を厚さ１０μｍ以下の保護膜でカバーしても良い。このような保護膜は、大気と音響整合層との直接的な接触を避け、音響整合層の特性を長期にわたって保持するのに寄与する。保護膜は、例えば、アルミニウム、酸化ケイ素、低融点ガラス、高分子などの材料からなる膜（単層に限定されない）によって構成される。保護膜は、スパッタリング法やＣＶＤ法などによって堆積される。
産業上の利用可能性
本発明によれば、音響インピーダンスが極めて低く、機械的強度の小さな材料から形成した薄い音響整合層を用いた高性能の超音波送受信器を実用化することが可能になる。使用時における信頼性も向上する。本発明の第１の態様では、音響整合層を保護するともに、音響整合層の厚さを規定するためにも利用可能な保護部を設けているため、機械的強度が低く、薄い音響整合層を高い精度で再現性良く形成することができる。その結果、広帯域の超音波を高感度で送受信できる信頼性の高い超音波送受波器が提供される。また、本発明の第２の態様では、音響整合層としても機能する保護部（保護部兼音響整合層＝保護整合層）を圧電体の主面に内の任意の位置に設けることができる。２種類の音響整合層の音速および厚さを調節することにより、厚さの異なる２種類の音響整合層から放射される超音波の位相を揃え、超音波の送受信感度を高めることが可能になる。
【図面の簡単な説明】
図１は、本発明の実施形態１における超音波送受信器を示す断面図である。
図２は、本発明の実施形態１における超音波送受信器の上面図である。
図３（ａ）は、本発明の実施形態１における超音波送受波器の送受信波形を示すグラフであり、図３（ｂ）は、従来の超音波送受波器の送受信波形を示すグラフである。
図４は、本発明の実施形態１において、音響整合層が収縮した場合を模式的に示す断面図である。
図５は、本発明の実施形態２における超音波送受信器の断面図である。
図６は、本発明の実施形態２における保護部の他の構成を示す断面図である。
図７は、本発明の実施形態２における保護部の他の構成を示す上面図である。
図８は、本発明の実施形態３における超音波送受信器の断面図である。
図９は、本発明の実施形態４における超音波送受信器の断面図である。
図１０は、本発明の実施形態５における超音波送受信器の断面図である。
図１１（ａ）は、本発明の実施形態５における超音波送受波器の送受信波形を示すグラフであり、図１１（ｂ）は、従来の超音波送受波器の送受信波形を示すグラフである。
図１２は、本発明の実施形態６における下層音響整合層の他の構成を示す断面図である。
図１３は、本発明の実施形態７における超音波送受信器の断面図である。
図１４は、本発明の実施形態７における超音波送受波器の送受信波形を示すグラフである。
図１５は、本発明の実施形態７における別構成の超音波送受信器の断面図である。
図１６（ａ）から図１６（ｃ）は、図１２に示す超音波送受波器の製造方法を示す工程断面図である。
図１７（ａ）は、図１６に示す製造工程において、ゲルの浸透が充分な場合の音響整合層を示す断面図であり、図１７（ｂ）は、ゲルの浸透が不充分な場合の音響整合層を示す断面図である。
図１８（ａ）から図１８（ｄ）は、図１２に示す超音波送受波器の他の製造方法を示す工程断面図である。
図１９（ａ）および図１９（ｂ）は、それぞれ、保護部の他の構成例を示す上面図である。
図２０は、本発明による超音波送受波器の第１０の実施形態の断面図である。
図２１は、本発明による超音波送受波器の第１０の実施形態の上面図である。
図２２は、本発明による超音波送受波器の第１０の実施形態における超音波の干渉を示す模式図である。
図２３は、保護整合層および音響整合層を伝搬した超音波の位相を模式的に示す断面図である。
図２４（ａ）から（ｃ）は、本発明による超音波送受波器の第１０の実施形態を製造する方法を示す工程断面図である。
図２５は、本発明による超音波送受波器の第１０の実施形態の送受信波形図である。
図２６は、本発明による超音波送受波器の第１１の実施形態の断面図である。
図２７は、本発明による超音波送受波器の第１１の実施形態の送受信波形図である。
図２８は、本発明による超音波送受波器の第１２の実施形態の断面図である。
図２９（ａ）から（ｄ）は、本発明による超音波送受波器の第１２の実施形態を製造する方法を示す工程断面図である。
図３０は、本発明による超音波送受波器の第１３の実施形態の断面図である。
図３１（ａ）および（ｂ）は、それぞれ、本発明による超音波送受波器の第１４の実施形態の上面図である。
図３２は、本発明による超音波送受波器の第１５の実施形態の断面図である。
図３３は、本発明の実施形態１６における超音波流量計を示すブロック図である。
図３４（ａ）および図２１（ｂ）は、本発明の超音波流量計で測定される波形を示すグラフである。
図３５は、従来の超音波流量計を示す断面図である。
図３６は、従来の超音波送受信器の断面図である。Technical field
The present invention relates to an ultrasonic transmitter / receiver having an acoustic matching layer, a manufacturing method thereof, and an ultrasonic flowmeter including the ultrasonic transmitter / receiver.
Background art
In recent years, ultrasonic flowmeters that determine the flow rate based on the moving speed by measuring the time during which the ultrasonic wave is transmitted for a predetermined distance in the pipe through which the fluid flows and measuring the moving speed of the fluid have been used in gas meters and the like. It's getting on.
FIG. 35 shows a cross-sectional configuration of the main part of this type of ultrasonic flowmeter. The ultrasonic flowmeter is arranged so that a fluid to be measured whose flow rate is to be measured flows in the pipe. A pair of ultrasonic transmitters / receivers 101a and 101b are installed on the tube wall 102 so as to face each other. The ultrasonic transceivers 101a and 101b are configured by using a piezoelectric vibrator such as a piezoelectric ceramic as an electric energy / mechanical energy conversion element, and exhibit resonance characteristics like a piezoelectric buzzer and a piezoelectric oscillator.
In the state shown in FIG. 35, the ultrasonic transceiver 101a is used as an ultrasonic transmitter, and the ultrasonic transceiver 101b is used as an ultrasonic receiver.
When an AC voltage having a frequency near the resonance frequency of the ultrasonic transmitter / receiver 101a is applied to a piezoelectric body (piezoelectric vibrator) in the ultrasonic transmitter / receiver 101a, the ultrasonic transmitter / receiver 101a functions as an ultrasonic transmitter, Ultrasound is emitted into the fluid. The emitted ultrasonic wave propagates to the path L1 and reaches the ultrasonic transceiver 101b. At this time, the ultrasonic transmitter / receiver 101b functions as a receiver, receives the ultrasonic wave, and converts it into a voltage.
Next, the ultrasonic transmitter / receiver 101b functions as an ultrasonic transmitter and the ultrasonic transmitter / receiver 101a functions as an ultrasonic receiver. That is, by applying an alternating voltage having a frequency near the resonance frequency of the ultrasonic transmitter / receiver 101b to the piezoelectric body in the ultrasonic transmitter / receiver 101b, ultrasonic waves are emitted from the ultrasonic transmitter / receiver 101b into the fluid. The emitted ultrasonic wave propagates along the path L2 and reaches the ultrasonic transceiver 101a. The ultrasonic transmitter / receiver 101a receives the propagated ultrasonic wave and converts it into a voltage.
As described above, since the ultrasonic transmitters / receivers 101a and 101b alternately function as a transmitter and a receiver, they are generally collectively referred to as an ultrasonic transmitter / receiver (or an ultrasonic transmitter / receiver). .
In the ultrasonic flow meter shown in FIG. 35, when an AC voltage is continuously applied, ultrasonic waves are continuously emitted from the ultrasonic transceiver and it becomes difficult to measure the propagation time. Is used as a drive voltage.
Hereinafter, the measurement principle of the ultrasonic flowmeter will be described in more detail.
When an ultrasonic burst signal is radiated from the ultrasonic transmitter / receiver 101a by applying a driving burst voltage signal to the ultrasonic transmitter / receiver 101a, the ultrasonic burst signal propagates through the path L1 and becomes ultrasonic transmitter / receiver after t time. 101b is reached. The distance of the route L1 is assumed to be L like the distance of the route L2.
The ultrasonic transceiver 101b can convert only the transmitted ultrasonic burst signal into an electrical burst signal with a high S / N ratio. This electric burst signal is electrically amplified and applied again to the ultrasonic transceiver 101a to radiate the ultrasonic burst signal. A device that performs such an operation is called a “sing-around type device”. In addition, the time until the ultrasonic pulse reaches the ultrasonic transmitter / receiver 102b after the ultrasonic pulse is emitted from the ultrasonic transmitter / receiver 101a is referred to as a “sing-around period”. The reciprocal of “sing-around period” is called “sing-around frequency”.
In FIG. 35, the flow velocity of the fluid flowing in the pipe is V, the velocity of the ultrasonic wave in the fluid is C, and the angle between the direction of flow of the fluid and the propagation direction of the ultrasonic pulse is θ. When the ultrasonic transmitter / receiver 101a is used as an ultrasonic transmitter / receiver and the ultrasonic transmitter / receiver 101b is used as an ultrasonic receiver, the time required for the ultrasonic pulse from the ultrasonic transmitter / receiver 101a to reach the ultrasonic transmitter / receiver 101b. Assuming that the sing-around period is t1 and the sing-around frequency f1, the following equation (1) is established.

Conversely, if the ultrasonic transmitter / receiver 101b is used as an ultrasonic transmitter and the ultrasonic transmitter / receiver 101 is used as an ultrasonic receiver, the sing-around period is t2, and the sing-around frequency f2 is The relationship (2) is established.

The frequency difference Δf between both sing-around frequencies is expressed by the following equation (3).

According to Expression (3), the flow velocity V of the fluid can be obtained from the distance L of the ultrasonic propagation path and the frequency difference Δf. The flow rate can be determined from the flow velocity V.
Such an ultrasonic flowmeter requires high accuracy. In order to increase accuracy, the acoustic impedance of the acoustic matching layer formed on the ultrasonic transmission / reception surface of the piezoelectric body in the ultrasonic transmission / reception device is important. The acoustic matching layer plays an important role particularly when the ultrasonic transceiver radiates (transmits) an ultrasonic wave to a gas and receives an ultrasonic wave that has propagated through the gas.
Hereinafter, the role of the acoustic matching layer will be described with reference to FIG. FIG. 36 shows a cross-sectional configuration of a conventional ultrasonic transceiver 103.
The illustrated ultrasonic transceiver 103 includes a piezoelectric body 106 fixed inside the sensor case 105 and an acoustic matching layer 104 fixed outside the sensor case 105. The acoustic matching layer 104 is bonded to the sensor case 105 with an epoxy adhesive or the like. Similarly, the piezoelectric body 106 is also bonded to the sensor case 105.
The ultrasonic vibration of the piezoelectric body 106 is transmitted to the sensor case 106 via the adhesive layer, and further transmitted to the acoustic matching layer 104 via another adhesive layer. Thereafter, the ultrasonic vibration is radiated as a sound wave to the gas (ultrasonic propagation medium) in contact with the acoustic matching layer 104.
The role of the acoustic matching layer 104 is to efficiently propagate the vibration of the piezoelectric body to the gas. Hereinafter, this point will be described in more detail.
The acoustic impedance Z of a substance is defined by the following equation (4) using the speed of sound C in the substance and the density ρ of the substance.

The acoustic impedance of the gas to be radiated by ultrasonic waves is greatly different from the acoustic impedance of the piezoelectric body. An acoustic impedance Z1 of a piezoelectric ceramic such as PZT (lead zirconate titanate) which is a general piezoelectric body is 30 × 10. ⁶ kg / m ² / S. On the other hand, the acoustic impedance Z3 of air is 400 kg / m. ² / S.
Sound waves are likely to be reflected at the boundary surfaces of substances having different acoustic impedances, and the intensity of the sound waves transmitted through the boundary surfaces is reduced. For this reason, a substance having an acoustic impedance Z2 represented by Expression (5) is inserted between the piezoelectric body and the gas.

When a substance having such an acoustic impedance Z2 is inserted, reflection at the boundary surface is suppressed, and sound wave transmittance is improved.
Acoustic impedance Z1 is 30 × 10 ⁶ kg / m ² / S, acoustic impedance Z3 is 400 kg / m ² / S, the acoustic impedance Z2 satisfying the equation (5) is 11 × 10 ⁴ kg / m ² / S. 11x10 ⁴ kg / m ² A substance having a value of / s must naturally satisfy the equation (4), ie Z2 = ρ × C. It is extremely difficult to find such substances from solid materials. The reason is that it is required that the density ρ is sufficiently small and the sound velocity C is low while being solid.
At present, as a material for the acoustic matching layer, a material obtained by solidifying a glass balloon or a plastic balloon with a resin material is widely used. Moreover, as a method for producing such a material over the acoustic matching layer, for example, Japanese Patent No. 2559144 discloses a method of thermally compressing a hollow glass sphere and a method of foaming a molten material.
However, the acoustic impedance of these materials is 50 × 10 ⁴ kg / m ² It is a value larger than / s, and it is difficult to say that the expression (5) is satisfied. In order to obtain a highly sensitive ultrasonic transmitter / receiver, it is necessary to form an acoustic matching layer with a material having a further reduced acoustic impedance.
In order to meet such a demand, the applicant of the present application has invented an acoustic matching material that sufficiently satisfies the expression (5), and disclosed it in the specification of Japanese Patent Application No. 2001-056051. This material is produced using a dry gel imparted with durability, has a low density ρ, and a low sound velocity C.
An ultrasonic transmitter / receiver including an acoustic matching layer formed of a material such as a dry gel having an extremely low acoustic impedance can efficiently transmit / receive ultrasonic waves to / from a gas. As a result, an apparatus capable of measuring the gas flow rate with high accuracy is realized.
However, materials with very low acoustic impedance, such as dry gels, generally have low mechanical strength. In particular, a dry gel is relatively strong against stress in the compression direction, but is extremely weak against stress in the tension and bending directions, and is easily broken by a weak impact.
In addition, since the sound speed of such a material is very slow, the thickness of an appropriate acoustic matching layer (about ¼ of the transmission / reception wavelength) for obtaining the maximum transmission / reception disturbance becomes very thin. For example, when the sound speed of the material is 60 to 400 m / s, when transmitting and receiving ultrasonic waves of about 500 kHz, the preferable thickness of the acoustic matching layer is about 30 to 200 μm. If it becomes thin like this, it is very difficult to handle the acoustic matching layer as a single member, and it is almost impossible to fabricate an ultrasonic transceiver by adhering the acoustic matching layer to the sensor case or piezoelectric body. Even if it is possible, it is difficult to put it to practical use from the viewpoint of manufacturing yield and cost.
Furthermore, due to the low mechanical strength of the acoustic matching layer, there is a possibility that the reliability of the acoustic matching layer may decrease due to the ultrasonic vibration itself inducing peeling of the acoustic matching layer during use as an ultrasonic transceiver.
The present invention has been made in view of the above problems, and the object of the present invention is to provide an acoustic matching layer formed of a material having low mechanical strength such as a dry gel and a slow sound speed, and can be manufactured with high yield. Another object of the present invention is to provide a highly reliable ultrasonic transmitter / receiver and a method for manufacturing the same.
Another object of the present invention is to provide an ultrasonic flowmeter provided with the above-described ultrasonic transceiver.
Disclosure of the invention
The ultrasonic transceiver according to the present invention includes a piezoelectric body, an acoustic matching layer provided on the piezoelectric body, and a position fixed to the piezoelectric body in contact with at least a part of a side surface of the acoustic matching layer. And a protective portion provided in the.
In a preferred embodiment, the protection portion protrudes in an ultrasonic radiation direction from a plane at the same level as the main surface of the piezoelectric body, and the height of the protection portion with respect to the main surface of the piezoelectric body is the acoustic level. Specifies the thickness of the matching layer.
In preferable embodiment, the said height of the said protection part is 5 micrometers or more and 2500 micrometers or less.
In a preferred embodiment, the thickness of the acoustic matching layer is substantially equal to the height of the protective part.
In a preferred embodiment, the thickness of the acoustic matching layer is about ¼ of the wavelength of the ultrasonic wave transmitted and / or received by the piezoelectric body.
In a preferred embodiment, the acoustic matching layer has a density of 50 kg / m. ³ 1000kg / m ³ It is formed from the following materials.
In a preferred embodiment, the acoustic matching layer has an acoustic impedance of 2.5 × 10 6. ³ kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² / S or less.
In a preferred embodiment, the acoustic matching layer is formed from an inorganic material.
In a preferred embodiment, the inorganic material is an inorganic oxide dry gel.
In a preferred embodiment, the inorganic oxide has a water-repellent solid skeleton.
In a preferred embodiment, the acoustic matching layer is solidified from a fluid state on the piezoelectric body provided with the protective portion.
In a preferred embodiment, a lower acoustic matching layer provided between the main surface of the piezoelectric body and the acoustic matching layer is provided, and the protection unit protrudes from the main surface of the second acoustic matching layer. The height of the protective part with respect to the main surface of the acoustic matching layer defines the thickness of the acoustic matching layer located in the uppermost layer.
In a preferred embodiment, the protection part is constituted by a part of the lower acoustic matching layer, and is integrated with the lower acoustic matching layer.
In preferable embodiment, the said height of the said protection part is 5 micrometers or more and 2500 micrometers or less.
In a preferred embodiment, the height of the protective part is substantially equal to the thickness of the acoustic matching layer located in the uppermost layer.
In a preferred embodiment, each of the first acoustic matching layer and the lower acoustic matching layer has a thickness of about ¼ of the wavelength of ultrasonic waves transmitted and received by the piezoelectric body.
In a preferred embodiment, the density of the first acoustic matching layer is 50 kg / m. ³ 1000kg / m ³ It is as follows.
In a preferred embodiment, the acoustic impedance of the lower acoustic matching layer is larger than the acoustic impedance of the first acoustic matching layer, and is 2.5 × 10. ³ kg / m ² / S or more 3.0 × 10 ⁷ kg / m ² / S or less.
In a preferred embodiment, the protection part is present on the outer periphery of the acoustic matching layer located in the uppermost layer.
In a preferred embodiment, the protection part covers the entire outer peripheral side surface of the acoustic matching layer located in the uppermost layer.
In a preferred embodiment, the protection portion is disposed outside the main surface of the piezoelectric body.
In a preferred embodiment, the protection part is provided on the main surface of the piezoelectric body.
In a preferred embodiment, the protection part is provided on the lower acoustic matching layer.
In a preferred embodiment, the protection part is constituted by a part of the lower acoustic matching layer, and is integrated with the lower acoustic matching layer.
In a preferred embodiment, a structural support for supporting the piezoelectric body is further provided.
In a preferred embodiment, a structural support for supporting the piezoelectric body is further provided, and the protection portion is provided on the structural support.
In a preferred embodiment, the structural support is made of press-molded metal, and the protection part is constituted by a portion bent by press-forming the structural support.
In preferable embodiment, the said height of the said protection part is 5 micrometers or more and 2500 micrometers or less.
In a preferred embodiment, the thickness of the acoustic matching layer is substantially equal to the height of the protective part.
In a preferred embodiment, the thickness of the acoustic matching layer is about ¼ of the wavelength of ultrasonic waves transmitted and received by the piezoelectric body.
In a preferred embodiment, the density of the acoustic matching layer is 50 kg / m. ³ 1000kg / m ³ It is as follows.
The acoustic impedance of the acoustic matching layer is 2.5 × 10 ³ kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² / S or less.
In a preferred embodiment, a back load material disposed on the back surface of the piezoelectric body is further provided, and the protection member is provided on the back load material.
In a preferred embodiment, the protection part is constituted by a part of the back load material and is integrated with the back load material.
In a preferred embodiment, at least a part of the surface of the acoustic matching layer that contacts the holding portion is subjected to a surface treatment for imparting a hydroxyl group.
In a preferred embodiment, at least a part of the surface where the acoustic matching layer contacts the holding portion is subjected to a roughening treatment.
In a preferred embodiment, at least a part of a surface where the acoustic matching layer is in contact with the holding portion is porous.
In a preferred embodiment, a portion of the acoustic matching layer penetrates and is integrated with a portion of the ultrasonic transceiver that is in contact with the acoustic matching layer.
Another ultrasonic transceiver of the present invention includes a piezoelectric body that performs ultrasonic vibration, and a density of 50 kg / m. ³ 1000kg / m ³ And the acoustic impedance is 2.5 × 10 ³ kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² Supporting an upper acoustic matching layer formed of a material of / s or less, a lower acoustic matching layer provided between the piezoelectric body and the upper acoustic matching layer, the lower acoustic matching layer and the piezoelectric body, An ultrasonic transmitter / receiver including a structural support that shields the piezoelectric body from an ultrasonic wave propagation fluid, and includes a protection unit that contacts at least a part of a side surface of the upper acoustic matching layer.
The protection part is formed by a part of the lower acoustic matching layer, and is integrated with the lower acoustic matching layer.
In a preferred embodiment, the elastic modulus of the protection part is substantially equal to the elastic modulus of the acoustic matching layer.
The ultrasonic flowmeter of the present invention includes a flow rate measurement unit through which a fluid to be measured flows, a pair of ultrasonic transmitters / receivers that are provided in the flow rate measurement unit and transmits / receives ultrasonic signals, and the pair of ultrasonic transmitters / receivers. An ultrasonic flowmeter comprising: a measuring means for measuring a time during which the ultrasonic wave propagates; and a flow rate calculating means for calculating a flow rate based on a signal from the measuring means, wherein the pair of ultrasonic transceivers Each is one of the above-described ultrasonic transceivers.
In a preferred embodiment, the piezoelectric body of the ultrasonic transceiver is shielded from the fluid to be measured.
The apparatus of the present invention includes any one of the above-described ultrasonic transceivers.
An ultrasonic transceiver manufacturing method according to the present invention includes a step (a) of preparing a piezoelectric body having a main surface and a convex portion received on the main surface, and acoustic matching on the main surface of the piezoelectric body. Forming a layer and contacting at least part of the side surface of the acoustic matching layer with the side surface of the convex portion.
In a preferred embodiment, the step (b) includes a step of supplying a gel raw material onto the main surface of the piezoelectric element, a step of forming the acoustic matching layer by drying and solidifying the gel raw material,
Is included.
In a preferred embodiment, the step (a) includes a step of processing the surface of the piezoelectric body to form the main surface and the convex portion.
In a preferred embodiment, the step (a) includes a step of fixing the convex portion on the surface of the piezoelectric body.
In a preferred embodiment, the step (a) includes a step of fixing the piezoelectric body to the structural support.
The method of manufacturing an ultrasonic transceiver according to the present invention includes an ultrasonic transceiver including an upper acoustic matching layer, a piezoelectric body, and a lower acoustic matching layer provided between the upper acoustic matching layer and the piezoelectric body. A method of manufacturing, comprising a step of preparing a member having a recess and functioning as the lower acoustic matching layer, and a step of supplying a gel raw material to the recess of the member (b), And (c) forming the upper acoustic matching layer by drying and solidifying the gel raw material.
In a preferred embodiment, the step (b) includes a step of allowing the gel material to penetrate into the member.
In a preferred embodiment, the gel raw material is allowed to penetrate the entire member.
In a preferred embodiment, the step (b) is performed after fixing the positional relationship between the member and the piezoelectric body. The step (b) may be performed before fixing the positional relationship between the member and the piezoelectric body.
The ultrasonic transmitter / receiver of the present invention includes a piezoelectric body, an acoustic matching layer provided on the piezoelectric body, and a protection unit disposed so as to be in contact with the outer peripheral surface of the acoustic matching layer.
The ultrasonic transmitter / receiver of the present invention is disposed so as to be in contact with a structural support, a piezoelectric body and an acoustic matching layer provided at positions facing each other with the structural support interposed therebetween, and an outer peripheral surface of the acoustic matching layer. And a protection unit.
Still another ultrasonic transducer according to the present invention includes a piezoelectric body having a main surface for transmitting and / or receiving ultrasonic waves, and an acoustic matching member provided on the main surface of the piezoelectric body. In the ultrasonic transducer, the acoustic matching member includes a first acoustic matching portion and a second acoustic matching portion having an average density lower than an average density of the first acoustic matching portion. The first acoustic matching portion is in contact with a side surface of the second acoustic matching portion.
In a preferred embodiment, the first acoustic matching portion is thicker than the second acoustic matching portion, is radiated from the main surface of the piezoelectric body, passes through the second acoustic matching portion, and is an upper surface of the first acoustic matching portion. The phase of the ultrasonic wave that has reached the same level position substantially coincides with the phase of the ultrasonic wave that has been radiated from the main surface and transmitted through the first acoustic matching portion and reached the upper surface of the first acoustic matching portion. ing
In a preferred embodiment, when the wavelength of the ultrasonic wave in the first acoustic matching portion is λ1, the thickness of the first acoustic matching portion is k1 · λ1 (k1 is 1/8 or more and 1/3 or less). And the thickness of the twenty-first acoustic matching portion is different.
In a preferred embodiment, the second acoustic matching portion includes N acoustic matching layers (N is an integer equal to or greater than 1), and each of the N acoustic matching layers includes the super acoustic layer in each acoustic matching layer. It has a magnitude of k2 times the wavelength of the sound wave (k2 is 1/8 or more and 1/3 or less).
In a preferred embodiment, the thickness of the acoustic matching layer located on the outermost layer of the second acoustic matching portion is about 1/4 of the wavelength of the ultrasonic wave in the acoustic matching layer located on the outermost layer of the second acoustic matching portion. It is.
In a preferred embodiment, the acoustic matching layer formed at a position closest to the main surface of the piezoelectric body in the second acoustic matching portion is made of the same material as the material of the first acoustic matching portion. .
In a preferred embodiment, an acoustic matching layer formed at a position closest to the main surface of the piezoelectric body in the second acoustic matching portion is formed integrally with the first acoustic matching portion.
In a preferred embodiment, at least one acoustic matching layer included in the second acoustic matching portion is formed of a dry gel.
In a preferred embodiment, the dry gel is made of an inorganic material.
In a preferred embodiment, the dry gel has a water-repellent solid skeleton.
In a preferred embodiment, at least a part of a surface in contact with the acoustic matching portion among the members constituting the acoustic wave transmitter / receiver is a rough surface or a porous surface.
In a preferred embodiment, among the members constituting the ultrasonic transducer, at least part of the surface in contact with the second acoustic matching portion, a part of the second acoustic matching portion is infiltrated and integrated with the member. .
In a preferred embodiment, at least a part of the second acoustic matching portion is made of a dry gel, and the first acoustic matching portion is made of a material having higher mechanical strength than the dry gel.
In a preferred embodiment, at least a part of the first acoustic matching portion is made of porous ceramics.
In a preferred embodiment, the thickness of the first acoustic matching portion changes according to the position on the main surface of the piezoelectric body.
In a preferred embodiment, the thickness of the second acoustic matching portion changes according to the position on the main surface of the piezoelectric body.
The ultrasonic flowmeter of the present invention includes a flow rate measurement unit through which a fluid to be measured flows, a pair of ultrasonic transducers that are provided in the flow rate measurement unit and transmit / receive ultrasonic signals, and the pair of ultrasonic transmission / reception units. An ultrasonic flowmeter comprising: a measuring unit that measures a time during which an ultrasonic wave propagates between devices; and a flow rate calculating unit that calculates a flow rate based on a signal from the measuring unit, wherein the pair of ultrasonic waves Each of the transducers is one of the above-described ultrasonic transducers.
In a preferred embodiment, the piezoelectric body of the ultrasonic transducer is shielded from the fluid to be measured.
In a preferred embodiment, the fluid to be measured is a gas.
The apparatus of the present invention includes any one of the above ultrasonic transducers.
The method for manufacturing an ultrasonic transducer according to the present invention includes (a) a first surface and a second surface opposite to the first surface, and electrodes are provided on the first and second surfaces. Preparing a formed piezoelectric body; (b) forming a second acoustic matching portion on at least one side of the first and second surfaces of the piezoelectric body; and (c) the piezoelectric body. Supplying a gel raw material into the space formed by the second acoustic matching portion, (d) obtaining a wet gel by gelling the gel raw material liquid, and (e) obtaining the wet gel Drying.
In a preferred embodiment, the step (c) includes: (c1) supplying a first gel raw material into the space; and (c2) gelling the first gel raw material liquid to form a first wet gel. Forming a layer; (c3) supplying a second gel raw material on the first wet gel layer; and (c4) gelling the second gel raw material liquid to form a second wet. Forming a gel layer, wherein the step (e) includes first and second wet gel layers, respectively, by drying the first and second wet gel layers. Forming a matching layer and a second acoustic matching layer.
In a preferred embodiment, in the step (c4), the first wet gel layer is modified so as to change an acoustic impedance of the first acoustic matching layer.
An ultrasonic transducer according to the present invention includes an ultrasonic wave including a piezoelectric body having a main surface for transmitting and / or receiving ultrasonic waves, and an acoustic matching member provided on the main surface of the piezoelectric body. In the transducer, the acoustic matching member includes a first acoustic matching portion and a second acoustic matching portion having a mechanical strength lower than the mechanical strength of the first acoustic matching portion, The first acoustic matching portion is in contact with the side surface of the second acoustic matching portion.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment 1)
FIG. 1 shows a section of an ultrasonic transceiver (ultrasonic transducer) in the first embodiment of the present invention. The illustrated ultrasound transmitter / receiver 1 includes a piezoelectric body 4, an acoustic matching layer 3 provided on the piezoelectric body 4, and a protection unit 2 fixed to the piezoelectric body 4.
The piezoelectric body 4 is made of a piezoelectric material and is polarized in the thickness direction. Electrodes (not shown) are formed on the upper and lower surfaces of the piezoelectric body 4 and emit ultrasonic waves based on signals applied to the electrodes. When receiving ultrasonic waves, a voltage signal is generated between the electrodes. In the present invention, the material of the piezoelectric body 4 is arbitrary, and a known material can be used.
The height H of the protective portion 2 with respect to the main surface (ultrasonic wave transmitting / receiving surface) S1 of the piezoelectric body 4 defines the thickness of the acoustic matching layer 3. In a preferred embodiment, the height of the protective portion 2 is acoustic. It is substantially equal to the thickness of the matching phase layer 3.
FIG. 2 shows the top surface of the ultrasonic transceiver 1 of FIG. As can be seen from FIG. 2, in the ultrasonic transceiver 1 of the present embodiment, the ring-shaped protection unit 2 surrounds the acoustic matching layer 3, and the entire outer peripheral surface (side surface) of the acoustic matching layer 3 is within the protection unit 2. It is in contact with the peripheral surface. By providing such a protective portion 2 on the upper surface of the piezoelectric body 4, the acoustic matching layer 3 is hardly peeled off from the piezoelectric body 4, and damage to the acoustic matching layer 3 can be prevented. As a result, the reliability of the ultrasonic transceiver 1 in the manufacturing stage and in the use stage is significantly improved.
In addition, according to the manufacturing method described later, the thickness of the acoustic matching layer 3 can be controlled with high accuracy by adjusting the height H of the protective portion 2. Therefore, since the acoustic matching layer 3 can be stably formed with high accuracy, an ultrasonic transmitter / receiver excellent in quality can be manufactured with high yield. If the thickness of the acoustic matching layer 3 varies from element to element, the characteristics (sensitivity, etc.) of the ultrasonic transmitter / receiver will vary, so it is important to form the acoustic matching layer 3 having a predetermined thickness with good reproducibility. . As described above, an appropriate acoustic matching layer thickness for obtaining the maximum transmission / reception sensitivity is about ¼ of the wavelength of ultrasonic waves to be transmitted / received. For this reason, when transmitting and receiving ultrasonic waves of about 500 kHz using a dry gel with a sound velocity of about 280 m / s as the acoustic matching layer, it is necessary to set the preferred thickness of the acoustic matching layer of the dry gel to about 140 μm. . If the thickness differs by about 10%, the transmission / reception sensitivity may vary by about 20%. As described above, the transmission / reception sensitivity greatly fluctuates only by a slight change in the thickness of the acoustic matching layer 3, but according to the present embodiment, the acoustic matching layer 3 having a desired thickness is formed with good reproducibility. Yes.
The ultrasonic transceiver 1 of this embodiment is manufactured as follows, for example.
First, a piezoelectric body 4 is prepared according to the wavelength of ultrasonic waves to be transmitted / received. As the piezoelectric body 4, a highly piezoelectric material such as piezoelectric ceramics or a piezoelectric single crystal is preferable. As the piezoelectric ceramic, lead zirconate titanate, barium titanate, lead titanate, lead niobate, or the like can be used. As the piezoelectric single crystal, lead zirconate titanate single crystal, lithium niobate, quartz, or the like can be used.
In this embodiment, lead zirconate titanate piezoelectric ceramics are used as the piezoelectric body 4, and the frequency of ultrasonic waves to be transmitted and received is set to 500 kHz. In order to allow the piezoelectric body 4 to efficiently transmit and receive such ultrasonic waves, the resonance frequency of the element is designed to be 500 kHz. For this reason, in this embodiment, the piezoelectric body 4 formed from a piezoelectric ceramic having a cylindrical shape with a diameter of 12 mm and a thickness of about 3 mm is used.
A ring-shaped protective portion 2 having an outer diameter of 12 mm, an inner diameter of 11 mm, and a thickness of 140 μm is bonded to such a piezoelectric body 4. In the present embodiment, a stainless metal ring is used as the protection unit 2. The stainless steel protective part 2 and the piezoelectric body 4 can be joined by bonding with an adhesive. For example, an epoxy resin may be used as an adhesive, and left for 2 hours in a thermostatic bath at 150 ° C. while applying a pressure of 0.2 MPa to be cured.
In the present embodiment, the acoustic matching layer 3 is formed from a dried gel. Since the acoustic velocity of the acoustic matching layer 3 formed from the dried gel is about 280 m / s, the wavelength of the ultrasonic wave in the acoustic matching layer 3 is about 640 μm. For this reason, the thickness of the acoustic matching layer 3 is set to 140 μm so as to be equal to about ¼ of the ultrasonic wavelength in the acoustic matching layer 3. In order to form the acoustic matching layer 3 having this thickness, in the present embodiment, the thickness of the protective portion 2 is set to 140 μm.
The role of the protection unit 2 is primarily to protect the acoustic matching layer 3 from external mechanical shocks or thermal shocks during the manufacturing and use stages of the ultrasonic transceiver 1. Secondly, it is also important to protect the ultrasonic transmitter / receiver 1 from vibration of ultrasonic waves to be transmitted / received during operation (use) as the ultrasonic transmitter / receiver 1.
In order for the acoustic matching layer 3 to fulfill its role, it is extremely important that the piezoelectric body 4 and the acoustic matching layer 3 are in close contact with each other. If even a slight separation occurs between the piezoelectric body 4 and the acoustic matching layer 3, it cannot function as the acoustic matching layer 3.
In order to maintain the adhesion between the piezoelectric body 4 and the acoustic matching layer 3, the inventor of the present application has a very effective structure in which the protective portion 2 is provided on the outer peripheral portion of the acoustic matching layer 3 as shown in FIG. I found something. When the protection unit 2 is not provided, there is a possibility that the characteristic deterioration is greatly progressed when the ultrasonic transmitter / receiver 1 is manufactured or used, and the high-performance ultrasonic transmitter / receiver 1 cannot be put into practical use.
The acoustic matching layer 3 of the present embodiment is formed of a material having an extremely small acoustic impedance defined by the product of the density ρ and the speed of sound C (ρ × C). For this reason, the transmission / reception efficiency of ultrasonic waves with respect to a gas such as air can be extremely increased. In the present embodiment, as described above, a dry gel is used as a material having an extremely low acoustic impedance.
By forming the acoustic matching layer 3 from a dry gel, the acoustic matching between the gas and the piezoelectric body is improved compared to the case where the acoustic matching layer is formed from a conventional material in which a glass balloon or a plastic balloon is solidified with a resin material. Since it becomes very good, ultrasonic transmission / reception efficiency can be remarkably improved.
The “dry gel” in the present specification is a porous body formed by a sol-gel reaction, and the solid skeleton portion solidified by the reaction of the gel raw material liquid passes through a wet gel composed of a solvent, It was formed by drying and removing the solvent. This dry gel is a nanoporous body in which continuous pores having an average pore diameter of several nanometers to several micrometers are formed by a nanometer-sized solid skeleton.
Since the porous body has such a fine structure, the sound velocity propagating through the solid portion becomes extremely small, and the sound velocity propagating through the gas portion inside the porous body due to the pores is extremely small. . Therefore, a very slow value of about 500 m / s or less is shown as the sound speed, and a low acoustic impedance completely different from that of the conventional acoustic matching layer can be obtained. In addition, since the nanometer-sized pores have a large pressure loss of gas, they have a feature that sound waves can be emitted with a high sound pressure when used as an acoustic matching layer.
As a material for such a dry gel, various materials such as an inorganic material and an organic polymer material can be used. As the solid skeleton portion of the inorganic material, silicon oxide (silica), aluminum oxide (alumina), titanium oxide, or the like can be used. As the solid skeleton portion of the organic material, a general thermosetting resin or a thermoplastic resin can be used. For example, polyurethane, polyurea, phenol curable resin, polyacrylamide, polymethyl methacrylate, or the like can be used. .
In the present embodiment, the acoustic matching layer 3 is formed from the above-described dry gel in the concave space P1 (see FIG. 1) formed in advance by the piezoelectric body 4 and the protection unit 2. That is, after the liquid gel material liquid is poured into the concave space P1 composed of the piezoelectric body 4 and the protection portion 2, gelation, hydrophobization, and drying are performed, so that the acoustic matching layer 3 is dried. Form a gel. In the present embodiment, a dry gel having a silicon oxide solid skeleton is used as the acoustic matching layer 3.
Specifically, the acoustic matching layer 3 can be formed by sequentially performing the following steps 1 to 4.
1. A gel raw material liquid (sol) prepared by preparing tetraethoxysilane, ethanol, and an aqueous ammonia solution (0.1 N) in a molar ratio of 1: 3: 4 is prepared.
2. This gel raw material liquid is dropped into a concave space formed by the piezoelectric body and the protective portion with a dropper. At that time, an excessive amount of the gel raw material liquid is dropped from the volume of the concave space P1. Next, a grinding operation using a Teflon (registered trademark) flat plate (not shown) was performed so that the height of the gel raw material liquid accumulated in the concave space P1 was the same as the height H of the protective part. Then, cover with a Teflon plate.
3. Leave at room temperature for about 1 day, and after the raw material solution has gelled (form wet gel), remove the Teflon plate. Thereafter, a hydrophobization treatment is performed in a 5% by weight hexane solution of trimethylethoxysilane.
4. It introduce | transduces into a supercritical drying tank, and supercritical drying is performed on the conditions of 12 Mpa and 50 degreeC under a carbon dioxide atmosphere. A dry gel is thus formed.
By such steps 1 to 4, for example, the density ρ is 0.3 × 10 ³ kg / m ³ The acoustic matching layer 4 having a sound velocity C of 280 m / s and a thickness of 140 μm can be formed.
The present invention has a density of 50 kg / m ³ 1000kg / m ³ And the acoustic impedance is 2.5 × 10 ³ kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² When the acoustic matching layer is formed from a material of / s or less, a remarkable effect is exhibited. However, according to the above method, such an acoustic matching layer can be preferably produced.
According to the above method, since the thickness of the acoustic matching layer 3 can be made substantially equal to the height H of the protective part 2, the thickness of the acoustic matching layer 3 can be controlled with high precision by the protective part 2. It can be said that the protection part 2 functions as a guide for the gel raw material liquid in a certain stage of the manufacturing process.
According to the above method, since the acoustic matching layer 3 with little thickness variation can be formed with a high yield, it is possible to suppress variations in characteristics of the ultrasonic transceiver. In the present invention, it is not essential to make the height of the protective portion 2 equal to the thickness of the acoustic matching layer 3. When the height of the protection part 2 is larger than the thickness of the acoustic matching layer 3, the function of suppressing the contraction of the acoustic matching layer 3 and protecting it from mechanical impact is sufficiently exhibited. Conversely, even if the height of the protective portion 2 is smaller than the thickness of the acoustic matching layer 3, if the protective portion 2 is not provided, the acoustic matching layer 3 can be prevented from contracting and mechanical shock can be prevented. The function to protect is exhibited to a high degree.
The transmission / reception waveform of the ultrasonic transmitter / receiver including the acoustic matching layer 4 manufactured by the above method was measured. A waveform diagram obtained by the measurement is shown in FIG. For comparison, FIG. 3B shows a transmission / reception waveform when a material in which a glass balloon is hardened with epoxy is used for an acoustic matching layer. The acoustic matching layer of the glass balloon used here has a density of 0.52 g / cm. ³ The sound speed is 2500 m / s and the thickness is 1.25 mm.
As described in the related art, it is desirable that the acoustic impedance of the acoustic matching layer has a value defined by Equation (5). In this embodiment, lead zirconate titanate piezoelectric ceramics are used for the piezoelectric body 4 and air is set as a propagation medium for propagating ultrasonic waves. Therefore, the density of the piezoelectric body 4 is 7.7 × 10. ³ kg / m ³ Since the sound speed is 3800 m / s, the acoustic impedance is about 29 × 10 ⁶ kg / m ² / S. On the other hand, the density of air is 0.00118 kg / m ³ Since the sound speed is 340 m / s, the acoustic impedance is about 0.0004 × 10 ⁶ kg / m ² / S. For this reason, from the equation (5), the preferable acoustic impedance of the acoustic matching layer is theoretically about 0.1 × 10 10. ⁶ kg / m ² / S.
The acoustic impedance of the acoustic matching layer 3 in the ultrasonic transceiver 1 of the present embodiment has a density of 0.3 × 10. ³ kg / m ³ Since the sound speed is 280 m / s, about 0.084 × 10 ⁶ kg / m ² / S, which is very close to the theoretical ideal value.
As can be seen from FIG. 3, according to this embodiment, it is possible to obtain a transmission / reception sensitivity that is three times or more that of a conventional sensor. Moreover, in this embodiment, since the protective part 2 is provided, even an ultrasonic transmitter / receiver having an acoustic matching layer formed from a dry gel that has low mechanical strength and is fragile can be manufactured with high yield, and even in use. It can continue to operate reliably for a long time. An external vibration test, a thermal shock test, a continuous vibration test, etc. were performed to evaluate whether or not the acoustic matching layer 3 peels from the piezoelectric body 4 under severe conditions. However, the performance of the ultrasonic transmitter / receiver deteriorates. There was no, and very stable operation was confirmed.
In the present embodiment, when the wet gel is dried to obtain a dry gel in the step 4, the supercritical drying method is used. However, drying in a normal atmosphere may be performed. In this case, contraction occurs in the process of changing from a wet gel to a dry gel, and a volume change of about 10 to 20% occurs. When such volume shrinkage occurs, the acoustic matching layer 3 peels from the piezoelectric body 4 in the conventional configuration. However, in the case of the present embodiment, as shown in FIG. 4, since the protective part 2 exists, the shrinkage of the dried gel mainly occurs only in the thickness direction. That is, almost no stress in the in-plane direction is generated at the interface between the piezoelectric body 4 and the acoustic matching layer 3, and peeling of the acoustic matching layer 3 is effectively prevented. Therefore, even if an air drying method that is simpler than the supercritical drying method is employed, the ultrasonic transceiver 1 with high sensitivity and high reliability can be manufactured, and the manufacturing cost can be reduced.
In addition, it is preferable that the height of the protection part 2 is set so that the final thickness of the acoustic matching layer 3 has an optimum size in consideration of the shrinkage rate of the dried gel. Note that it is not preferable that the degree of shrinkage becomes excessively large and the thickness of the thinnest portion of the acoustic matching layer 3 decreases to 90% or less of the average thickness, because the characteristics of the acoustic matching layer 3 deteriorate. According to the supercritical drying method, the thickness of the thinnest portion of the acoustic matching layer 3 can be maintained at 98% or more of the average thickness.
Surface treatments such as plasma cleaning and acid treatment are performed in advance on the upper surface of the piezoelectric body 4 in contact with the lower surface of the acoustic matching layer 3 and the inner surface of the protective portion 2 in contact with the side surface of the acoustic matching layer 3. Is preferred. When a hydroxyl group is formed on the contact surface by such treatment, the chemical bond between the dried gel, the piezoelectric body 4 and the protective portion 2 can be further strengthened.
In order to realize a strong coupling between the acoustic matching layer 3 and the piezoelectric body 4 and the protection unit 2, a region in contact with the acoustic matching layer 3 may be roughened on the surface of the piezoelectric body 4 and the protection unit 2. . As a roughening method, normal file, blasting, physical or chemical etching, etc. can be used effectively.
In order to improve the adhesion between the acoustic matching layer 3 and the protection part 2, it is also effective to use a porous material as the material of the protection part 2. By forming the protection part 2 from the porous body, a part of the acoustic matching layer 3 penetrates into the protection part 2 and is integrated, so that a stronger adhesion state can be obtained.
Examples of the porous body that can be used for the protective part 2 include metals, ceramics, and resins manufactured by a foaming method or the like. Various materials such as stainless steel, nickel, and copper can be used as the porous metal, alumina and barium titanate can be used as the ceramic, and epoxy and urethane can be used as the resin.
In this specification, “protecting the acoustic matching layer” not only protects the acoustic matching layer from mechanical vibrations and shocks, but also creates a sound matching layer from a material that shrinks during formation. It also includes suppressing the peeling of the matching layer. By adopting a member that protects the acoustic matching layer in this way, even if the acoustic matching layer is formed from a material having low mechanical strength and shrinkage, the function of the acoustic matching layer (piezoelectric It is possible to maintain a practical level of operation that can efficiently transmit and receive ultrasonic waves between the body and the ultrasonic propagation medium.
(Embodiment 2)
A second embodiment of the present invention will be described with reference to FIG.
In the present embodiment, the protection part and the piezoelectric body are integrated. Specifically, a recess 5a is formed in the central portion of the main surface of the piezoelectric body 5, and a part 5b of the piezoelectric body 5 is used as a protective portion. In other words, a part 5b of the piezoelectric body 5 functions as a protection portion, and the protection portion and the piezoelectric body are integrally formed.
The ultrasonic transceiver of FIG. 5 is manufactured as follows.
First, after preparing a piezoelectric body 5 that has been subjected to polarization treatment, one surface (main surface) of the piezoelectric body 5 is processed to form a recess 5a. The processing for forming the recess 5a can be performed by an end mill or sand blasting. The depth of the recess 5a corresponds to the height of the protection part (5b). Thereafter, an electrode is formed on the surface where the recess 5a is formed, and an electrode is also formed on the surface opposite to the recess of the piezoelectric body. The electrode is produced by forming a metal film such as gold or nickel by plating or sputtering, for example.
According to the present embodiment, since the piezoelectric body 5 is processed and the peripheral portion of the main surface functions as a protective portion, a step of joining a separately manufactured protective portion to the piezoelectric body becomes unnecessary. When bonding is used for bonding of the protective part, it is necessary to consider the change in the height of the protective part due to the presence of the adhesive layer, but according to this embodiment, the protective part whose height is defined with high accuracy Therefore, the thickness of the acoustic matching layer 3 can be adjusted with high accuracy, and a high-performance ultrasonic transceiver can be provided stably.
Also in this embodiment, it is preferable to perform a process of forming a hydroxyl group on a portion of the surface of the piezoelectric body 5 that is in contact with the acoustic matching layer 3. Further, when the piezoelectric body 5 is roughened during the process of forming the recess 5 a in the piezoelectric body 5, the adhesion between the acoustic matching layer 3 and the piezoelectric body 5 can be further improved.
In the first embodiment and the second embodiment, the portion functioning as the protection portion is formed in a ring shape having a side surface perpendicular to the main surface of the piezoelectric body 5. However, the side surface of the protection portion may have a taper as shown in FIG. Moreover, as shown in FIG. 7, it is not necessary to be in contact with all of the outer peripheral side surface of the acoustic matching layer 3, and has a structure divided into a plurality of parts or a structure in which notches are formed in part. May be.
According to the configuration of the first or second embodiment described above, even when a low-density material such as a dry gel and a slow sound speed is used for the acoustic matching layer, the protective unit strengthens the coupling between the acoustic matching layer and the piezoelectric body. Thus, it is possible to provide a high-performance ultrasonic transmission transducer with a high yield, while exhibiting high transmission / reception sensitivity and facilitating handling in the manufacturing process stage of the ultrasonic transmission transducer. In addition, an element having excellent reliability that is less likely to deteriorate due to mechanical shock at the stage of using the ultrasonic transducer and vibration caused by transmission / reception of ultrasonic waves is realized.
(Embodiment 3)
A third embodiment of the present invention will be described with reference to FIG.
A characteristic point of this embodiment is that the structure support 6 is provided. The structural support 6 shown in FIG. 8 includes a disc-like support portion 6a to which the acoustic matching layer 3 is fixed, and a cylindrical portion 6b extending continuously from the disc-like support portion in the axial direction. The end portion of the cylindrical portion is bent in an L shape in cross section, and is easily fixed to a shielding plate 60 or another device.
The acoustic matching layer 3 and the protection part 2 are arranged on the surface of the support part 6a of the structural support body 6, and the piezoelectric body 4 is arranged on the back surface of the support part 6a. That is, the piezoelectric body 4 and the acoustic matching 3 are respectively provided at positions facing each other with the structure support 6 interposed therebetween. By using such a structure support body 6, handling of an ultrasonic transmitter / receiver (ultrasonic transmitter / receiver) becomes extremely easy.
The structural support 6 can be constituted by a container (sensor case) that can be sealed. In this case, if the open end of the cylindrical portion 6b of the structural support 6 is closed with a shielding plate 60 or the like, and the inside of the structural support 6 is filled with an inert gas, the piezoelectric body 4 is removed from the fluid to be measured for flow velocity. Can be cut off. Since voltage is applied to the piezoelectric body 4, if the piezoelectric body 4 is surrounded by a combustible gas, there is a risk of applying it to the combustible gas. However, since the structural support 6 is composed of a sealed container and the inside is shut off from the outside, such an ignition can be prevented, so that ultrasonic waves can be safely emitted even to a combustible gas. . Even if the external gas is not flammable, the piezoelectric body 4 is shielded from the external gas even when the ultrasonic wave is emitted to the gas that may react with the piezoelectric body 4 and possibly deteriorate the characteristics of the piezoelectric body 4. By doing so, it is possible to suppress the deterioration of the piezoelectric body 4 and realize a highly reliable operation over a long period of time.
In the example of FIG. 8, the protection unit 2 is disposed on the outer peripheral portion of the ultrasonic transmission / reception surface of the piezoelectric body 4. In general, since the protection unit 2 does not play the role of the acoustic matching layer 4, when the protection unit 2 is disposed on the main surface of the piezoelectric body 4, the portion does not contribute to transmission / reception of ultrasonic waves, and transmission / reception sensitivity decreases. End up.
In order to prevent the structural support body 6 from being an acoustic obstruction factor, it is desirable that the thickness of the disk-shaped support portion 6a with which the piezoelectric body 4 contacts is 1/8 or less of the wavelength of ultrasonic waves to be transmitted and received. . By making this thickness about 1/8 or less of the wavelength, the structural support 6 does not hinder the propagation of ultrasonic waves.
In the present embodiment, stainless steel is used as the material of the structural support 6 and the thickness of the structural support 6 is set to 0.2 mm. Since the speed of sound in stainless steel is about 5500 m / s, 0.2 mm corresponds to about 1/55 of the wavelength in an ultrasonic wave having a frequency of 500 kHz. Since the structural support 6 is formed of such thin stainless steel, even if the structural support 6 is interposed in the ultrasonic wave propagation path, there is almost no acoustic obstacle.
The material of the structural support 6 is not limited to a metal material, and a material according to the purpose can be selected from ceramic, glass, and resin. In the present embodiment, the external fluid and the piezoelectric body are reliably separated, and the strength that can prevent the contact between the piezoelectric body and the external fluid is given even if some mechanical shock is applied to the structural support 6. Therefore, the structural support 6 is produced from a metal material. Thereby, for example, even when ultrasonic waves are transmitted / received for flammable or explosive gases, high safety can be ensured.
When ultrasonic waves are transmitted / received to / from a safe gas, the structural support 6 may be formed from a material such as a resin for the purpose of cost reduction.
In order to improve the adhesion between the structural support 6 and the acoustic matching layer 3, plasma treatment or acid treatment for adding a hydroxyl group is performed in advance on the surface of the structural support 6 in contact with the acoustic matching layer 3. Is preferred. Further, this portion may be roughened by sanding, sandblasting, chemical and / or physical etching, or the like.
(Embodiment 4)
Next, a fourth embodiment of the present invention will be described with reference to FIG.
In the ultrasonic transceiver according to this embodiment, a part 7a of the structure support 7 functions as a protection part, and the structure support 7 and the protection part are integrated. For example, when the structural support 7 is produced by press molding a metal material such as stainless steel, the concave portion 7b is formed in the disk-shaped support portion, and the periphery of the concave portion 7b (bent by the press molding of the structural support 7 is formed. Part 7a) can be used as a protective part.
By employ | adopting such a structure, the process of joining a protection part to a structure support body can be skipped. Further, similarly to the first embodiment, the height of the protective portion does not vary due to the adhesive layer, so that a highly sensitive ultrasonic transceiver can be manufactured with high yield.
In addition, in FIG. 9, although the plate for sealing the structure support body 7 is not described, you may adhere or integrate such a plate to the structure support body 7 as needed. The same applies to other embodiments described below.
(Embodiment 5)
A fifth embodiment of the present invention will be described with reference to FIGS. 10 and 11.
The ultrasonic transmitter / receiver of this embodiment includes another acoustic matching layer (lower acoustic matching layer) 8 disposed between the acoustic matching layer 3 and the piezoelectric body 4. Except for the insertion of the lower acoustic matching layer 8, the configuration of the present embodiment is the same as the configuration of the second embodiment.
The acoustic matching layer plays a role of suppressing the internal reflection of the sound wave due to the mismatch of the acoustic impedance and efficiently radiating the ultrasonic wave from the piezoelectric body to the medium (ultrasonic propagation medium). One layer is sufficient for such an acoustic matching layer when transmitting or receiving ultrasonic waves having a single frequency (continuous wave ultrasonic waves).
In contrast, a normal ultrasonic transmitter / receiver transmits and receives pulsed or bursty ultrasonic waves. Pulsed or bursty ultrasonic waves contain broadband frequency components rather than a single frequency component. In order to transmit and receive such ultrasonic waves with high sensitivity, it is preferable that the acoustic impedance of the acoustic matching layer is gradually changed between the piezoelectric body and the ultrasonic propagation medium. In order to gradually change the acoustic impedance in this way, the acoustic matching layer may be multilayered and the acoustic impedance of the constituent layers may be gradually shifted.
In the present embodiment, as shown in FIG. 10, the acoustic matching layer is made into two layers. Specifically, a porous sintered body made of ceramics is used as the lower acoustic matching layer 8. This acoustic matching layer 8 has an apparent density of about 0.64 × 10 6. ³ kg / m ³ , Sound speed is 2000m / s, acoustic impedance is about 1.28 × 10 ⁶ kg / m ² / S. As the ceramic, a barium titanate-based material is used.
The “apparent density” is a density including a space portion included in the porous body. In the porous ceramics, about 80% of the volume is a space part (hole), and the solid part of the ceramic is about 20% by volume of the whole. Such porous ceramics are formed by mixing and press-molding resin balls and ceramic powder, and then heating and burning away the resin balls in the process of sintering the ceramics. If heating is performed rapidly during sintering, the resin balls expand or rapidly gasify, and the ceramic structure is destroyed. Therefore, it is preferable to perform gentle heating.
In the present embodiment, such a lower acoustic matching layer 8 is fixed to the piezoelectric body 4 (directly the structural support 6), and then the acoustic matching layer 8 on the side opposite to the piezoelectric body 4. The protective part 2 is joined to the surface. The protection part 2 can use what was produced from the ring made from stainless steel similarly to the protection part 2 used in Embodiment 1. FIG. All bonding can be performed with an epoxy adhesive.
A dry gel layer to be the acoustic matching layer 3 was formed in the recess formed by the lower acoustic matching layer 8 and the protective part 2 in the same manner as in the first embodiment.
In the present embodiment, in Step 1 similar to Step 1 in Embodiment 1, the density of the dried gel is adjusted by changing the concentration of ammonia serving as the gelation reaction catalyst, and the density of the acoustic matching layer is 0.2 ×. 10 ³ kg / m ³ , Sound speed 160m / s, acoustic impedance about 0.032 × 10 ⁶ kg / m ² A / s dry gel layer is formed. Since the sound velocity of the acoustic matching layer is 160 m / s, the height of the protection unit 2 is set to 80 μm so as to be 1/4 of the ultrasonic wavelength in the acoustic matching layer. It is preferable to perform the process which provides a hydroxyl group to the inner surface of the protection part 2 by plasma etching.
FIG. 11A shows a transmission / reception waveform of the ultrasonic transceiver according to the present embodiment. In the graph of FIG. 11, the vertical axis represents signal amplitude and the horizontal axis represents time. The numerical value on the axis is an index notation, for example, “2.0E-04” is 2.0 × 10 ^-4 Means. The same applies to the other graphs.
In the ultrasonic transmitter / receiver used for the measurement, the thickness of the porous ceramic to be the lower acoustic matching layer 8 is set to 1 mm, and the thickness of the protective portion 2 and the first acoustic matching layer (dry gel layer) 2 is set to 80 μm. did.
For comparison, an ultrasonic transmitter / receiver using a conventional acoustic matching layer in which a glass balloon is hardened with epoxy instead of the two acoustic matching layers 3 and 8 in the ultrasonic transmitter / receiver of FIG. Transmitted and received waveforms were measured. The measurement results are shown in FIG.
By using two acoustic matching layers, a sensitivity about 20 times higher than that of a conventional ultrasonic sensor could be obtained. Also. Even when compared with the ultrasonic sensor according to the first embodiment, it can be seen that the high sensitivity and the wide band (short pulse) were achieved at the same time. As described above, by making the acoustic matching layer into two layers, it is possible to provide an ultrasonic transceiver that is extremely suitable for transmitting and receiving pulses and burst waves.
(Embodiment 6)
A sixth embodiment of the present invention will be described with reference to FIG.
In this embodiment, a part of the lower acoustic matching layer 9 functions as a protective part, and the acoustic matching layer 9 and the protective part are integrated. In this example, the acoustic matching layer 9 is processed to form a recess on its main surface. The dry gel layer to be the upper acoustic matching layer 3 is formed in the recess of the lower acoustic matching layer 9.
By employ | adopting such a structure, the process of joining a protection part to the acoustic matching layer 9 can be skipped. In addition, the problem that the height of the protective portion varies due to the adhesive layer can be solved, and an ultrasonic transceiver that operates with high sensitivity in a wide band can be manufactured with high yield.
In the present embodiment, the lower acoustic matching layer 9 is formed of a porous body. For this reason, the coupling | bonding with the upper acoustic matching layer 3 is strong, and it can ensure high sensitivity and high stability. In order to further improve the adhesion, it is preferable to perform plasma treatment or acid treatment for imparting a hydroxyl group to the contact surface between the upper acoustic matching layer 3 and the lower acoustic matching layer 9 in advance.
In this embodiment and Embodiment 5 described above, the acoustic matching layer has a two-layer structure, but the ultrasonic transducer of the present invention is not limited to such a configuration, and has a multilayer structure of three or more layers. You may have. By multilayering the acoustic matching layer, the sensitivity can be further increased and the band can be widened. However, in order to increase the sensitivity by increasing the number of layers, it is necessary to form an acoustic matching layer that is the outermost layer from a material having an extremely low acoustic impedance. Therefore, it is practical to adopt a two-layer structure.
(Embodiment 7)
The seventh embodiment of the present invention will be described with reference to FIGS.
As shown in FIG. 13, the ultrasonic transmitter / receiver according to the present embodiment has a back load material 10 bonded to the back side of the piezoelectric body 4, and the protection unit 2 is formed above the back load material 10. In other respects, the configuration is the same as that of the third embodiment.
The back load material 10 has a function of attenuating ultrasonic waves radiated from the piezoelectric body 4 to the back side, and any material can be used as long as it can exhibit such a function. Also good.
The protection part 2 is made of a cylindrical metal and is bonded to the main surface of the back load material 10. The thickness of the protection unit 2 is equal to the total thickness of the piezoelectric body 4, the lower acoustic matching layer 8, and the upper acoustic matching layer 3. In the present embodiment, since the thickness of the piezoelectric body 4 is 3 mm, the thickness of the acoustic matching layer 8 is 1 mm, and the thickness of the acoustic matching layer 3 is 0.08 mm, the thickness of the protective portion 2 is 4 0.08 mm.
The back load material 10 of this embodiment is formed from ferrite rubber. Ferrite rubber is a material in which iron powder is dispersed in rubber, and has a high sound wave attenuation rate. By bonding such a back surface load material 10 to the back surface of the piezoelectric body 4, the ultrasonic wave radiated from the back surface side of the piezoelectric body 4 is attenuated, and broadband (short pulse width) ultrasonic waves are transmitted and received. Is possible.
FIG. 14 shows a transmission / reception waveform measured for the ultrasonic transceiver having the configuration of FIG. Although the transmission / reception sensitivity of this embodiment is lower than that of the ultrasonic transmitter / receiver of the third embodiment, an operation in a wider band is realized, and an ultrasonic transmitter / receiver suitable for transmitting / receiving short pulses is configured can do.
Instead of the back load material 10 shown in FIG. 13, the back load material 11 shown in FIG. 15 may be used. A portion of the back load material 11 shown in FIG. 15 functions as a protection portion, and the protection portion and the back load material are integrated. The back load material 11 has a configuration in which a recess is formed in a region excluding the peripheral portion of the main surface, and the piezoelectric body 4 is inserted into the recess and is bonded to the inner surface of the recess of the back load material 11. The depth of the concave portion provided in the back load material 11 is set to be larger than the height of the piezoelectric body 4, and if a dry gel layer serving as an acoustic matching layer is formed on the upper surface of the piezoelectric body 4 after insertion, FIG. 15 configurations are obtained. By adopting the back load material 11, the same broadband as the ultrasonic transmitter / receiver of FIG. 13 is achieved.
(Embodiment 8)
With reference to FIG. 16, an embodiment of a method of manufacturing the ultrasonic transceiver shown in FIG. 12 will be described.
First, as shown in FIG. 16A, the piezoelectric body 4 and the lower acoustic matching layer 9 are joined to the structural support 6. An adhesive can be used for joining. As described above, the piezoelectric body 4 is made of piezoelectric ceramics, and the structural support 6 is made of stainless steel. The lower acoustic matching layer 9 is made of porous ceramics having a recess on the upper surface. The concave portion is formed by processing the upper surface of a flat porous ceramic with a lathe or the like.
Next, as shown in FIG. 16 (b), a dry gel to be an upper acoustic matching layer is formed in the concave portion of the acoustic matching layer 9 adhered to the structural support 6. The formation of the dried gel can be performed by the method described in the first embodiment.
In order to sufficiently infiltrate the gel raw material into the acoustic matching layer 9 formed of porous ceramics, it is preferable to place the gel raw material in a vacuum or a reduced pressure atmosphere after pouring the gel raw material. In this way, in the present embodiment, the gel raw material is allowed to penetrate not only the portion functioning as the protective portion of the acoustic matching layer 9 but also the entire interior of the acoustic matching layer 9. By doing so, the dried gel can be firmly bonded to the lower acoustic matching layer 9 and the characteristics of the acoustic matching layer 9 can be made uniform (FIG. 16C). Hereinafter, this point will be described with reference to FIG.
FIG. 17A shows a state where the gel raw material has sufficiently penetrated into the lower acoustic matching layer 9. Therefore, the lower acoustic matching layer 9 functions acoustically as a single layer, and the acoustic impedance is stepped from a relatively high value of the acoustic matching layer 9 to a relatively low value of the upper acoustic matching layer. It decreases to the shape.
On the other hand, when the penetration of the gel raw material is insufficient, as shown in FIG. 17 (b), an acoustic matching layer having a substantially three-layer structure is formed. In this case, the acoustic impedance of the lowermost layer (first layer) where the penetration of the gel raw material is insufficient is smaller than the set value, so that the acoustic impedance of the middle layer (second layer) is the largest. When the distribution of the acoustic impedance is as shown in FIG. 17B, the acoustic impedance is not reduced stepwise from the piezoelectric body (not shown) arranged in the lower part of the figure toward the gas as the ultrasonic propagation medium. In addition, since the characteristics of the ultrasonic transducer are deteriorated, it is preferable to sufficiently permeate the gel raw material.
In the above embodiment, as shown in FIG. 16, after the step of fixing the lower acoustic matching layer 9 to the structural support 6, the step of forming the first acoustic matching layer 3 is performed. The order may be reversed. With reference to FIGS. 18A to 18D, another manufacturing method will be described.
First, as shown in FIG. 18A, an acoustic matching layer 9 having a portion that functions as a protection portion is prepared. Next, as shown in FIG. 18 (b), the gel raw material is dropped into the concave portion of the acoustic matching layer 9, and the gel raw material is ground so that the height of the gel raw material in the concave portion matches the height of the protective portion. It penetrates the entire acoustic matching layer 9. After the gel material is cured and hydrophobized, the gel raw material is dried by a supercritical drying method, and the acoustic matching layer 3 made of the dried gel is formed on the lower acoustic matching layer 9 as shown in FIG. .
Thereafter, as shown in FIG. 18D, the acoustic matching layer is bonded to the structure support 6 in a state where the piezoelectric body 4 is fixed. Note that the acoustic matching layer in FIG. 18C may be directly bonded to the piezoelectric body 4 without using the structural support 6.
In addition, it is preferable to select an optimal pressurizing condition so that the dried gel is not destroyed by pressurization during bonding. Since the strength of the dried gel with respect to the stress in the compression direction is relatively high, the production yield hardly decreases in the bonding step.
The materials of the acoustic matching layer 3 and the acoustic matching layer 9 are preferably selected so that the elastic modulus of the upper acoustic matching layer 3 and the elastic modulus of the lower acoustic matching layer 9 are close to each other. When the elastic moduli of both are close to each other, a uniform pressure can be applied to the entire bonding surface, and it becomes easy to manufacture an ultrasonic transmitter / receiver with high sensitivity at a high yield.
In the method shown in FIGS. 18A to 18D, since it is not necessary to handle the piezoelectric body and the structural support in the process of forming the dry gel on the acoustic matching layer 9, the equipment such as the drying apparatus is small. This makes it possible to manufacture an ultrasonic transducer at a low cost.
In the dry gel formation process, there is a possibility that a chemical load may be applied to organic substances such as an adhesive layer. However, the adhesion part is not deteriorated by performing the adhesion process after the formation of the dry gel.
(Embodiment 9)
Another embodiment of the present invention will be described with reference to FIG.
A characteristic point of the present embodiment is that the protective portion 2 is formed not only in the outer peripheral portion of the region where the acoustic matching layer 3 is formed, but also in the region.
When forming a dry gel layer from a gel raw material through a wet gel, irregularities may be formed on the upper surface of the dry gel layer. Further, when the wet gel is dried by a normal drying method without using supercritical drying, the shrinkage of the dry gel occurs, so that a recess as shown in FIG. 4 is easily formed in the dry gel.
When the ultrasonic transmission / reception surface is wide, the above irregularities tend to be large, and even if the thickness of the acoustic matching layer is set to the optimum value, the thickness of the acoustic matching layer is actually increased from the optimum value depending on the location. It will shift.
Since the speed of sound in the dry gel layer is extremely slow, it is necessary to form a very thin layer to function properly as an acoustic matching layer, and the allowable error range of the thickness is small.
If a protective portion having a layout as shown in FIG. 19A or FIG. 19B is formed, an error in the thickness of the acoustic matching layer can be suppressed to within about ± 5% from the target value.
As shown in FIG. 19 (a) or FIG. 19 (b), when the protective part 2 is also provided inside the ultrasonic radiation surface of the ultrasonic transceiver, the variation in the thickness of the acoustic matching layer 3 is minimized. However, the protection unit 2 provided on the inner side of the ultrasonic radiation surface can be an obstacle to transmission / reception of ultrasonic waves. In order to prevent acoustic characteristics from being deteriorated by the protection unit 2, it is preferable that the size of the protection unit 2 be as small as possible in a range that serves as the protection unit 2.
In the configuration example of FIG. 19A, the protective portions 2 having a circular cross section are randomly arranged. However, the cross sectional shape of the protective portion 2 is not limited to a circular shape, and may be a rectangle or a polygon. Further, the arrangement is not limited to a random arrangement.
In the configuration example of FIG. 19B, the concentric protection part 2 is provided. In this configuration example, the protection unit 2 is also present inside the ultrasonic radiation surface of the ultrasonic transceiver, but the characteristic deterioration of the ultrasonic transceiver is prevented. The configuration of FIG. 19B is effective when transmitting an ultrasonic wave to a position separated from the ultrasonic radiation surface by a short distance L on the central axis of the ultrasonic handset.
When the distance between the protection unit 2 and the center of the ultrasonic radiation surface is r, the distance r preferably satisfies the following formula (6).

Here, λ is the wavelength in the gas through which the ultrasonic wave propagates, and L is the distance from the ultrasonic radiation surface of the ultrasonic transceiver. For example, when the frequency is 500 kHz, the ultrasonic propagation medium is air (sonic velocity 340 m / s), and the measurement distance L is 10 mm, the radius r from the center is 2.6 to 3.7 mm, 4.6 from Equation (6). It turns out that it is preferable to provide the protection part 2 in the position of -5.4 mm, 6.1-6.7 mm .... Providing the protection unit 2 at such a position is effective in preventing disturbance of the sound field due to sound interference and preventing deterioration in sensitivity of ultrasonic waves at a short distance.
When each point on the sound wave emitting surface of the ultrasonic transceiver is regarded as a point sound source, a synthesized ultrasonic wave from each point sound source becomes an ultrasonic wave to be transmitted. When the distance from the ultrasonic radiation surface is short, ultrasonic waves having different phases cancel each other, and there is a position where high-power ultrasonic waves cannot be transmitted. In order to radiate only ultrasonic waves having the same phase from the ultrasonic radiation surface, it is effective to provide the protection unit 2 in a region where ultrasonic waves having different phases are radiated. When the protection unit 2 is provided in such a region, it is possible to suppress the emission of ultrasonic waves having different phases, so that the disturbance of the sound field at a short distance can be prevented and high-power ultrasonic transmission can be performed.
(Embodiment 10)
FIG. 20 is a sectional view showing a tenth embodiment of the ultrasonic transducer according to the present invention. The ultrasonic transducer 21 of this embodiment includes a piezoelectric body 22, electrodes 23a and 23b provided on both surfaces of the piezoelectric body 22, and a protective matching layer (first layer) provided on the piezoelectric body 22 via the electrode 23a. 1 acoustic matching portion) 4 and an acoustic matching layer (second acoustic matching portion) 25 provided on the piezoelectric body 22 via an electrode 23a.
FIG. 21 is a top view of the ultrasonic transducer 21 shown in FIG. As can be seen from FIG. 21, the ultrasonic transducer of this embodiment has a structure in which protective matching layers 24 and acoustic matching layers 25 having different thicknesses (heights) are alternately arranged in a concentric manner. Yes.
The piezoelectric body 22 in the present embodiment is formed of a material having piezoelectricity and is polarized in the thickness direction. When a voltage is applied to the electrodes 23 a and 23 b provided on the upper and lower surfaces of the piezoelectric body 22, an ultrasonic wave is generated in the piezoelectric body 22 based on the voltage signal, and the ultrasonic wave is transmitted through the protective matching layer 24 and the acoustic matching layer 25. Radiated to a sound wave propagation medium (gas or the like) 26. Further, the ultrasonic wave propagated through the ultrasonic propagation medium 26 propagates to the piezoelectric body 22 through the protective matching layer 24 and the acoustic matching layer 25. The piezoelectric body 22 is deformed by the incident ultrasonic wave, and a voltage signal is generated between the electrode 23a and the electrode 23b.
The material of the piezoelectric body 22 is arbitrary, and those formed from various known materials can be used. A known electrostrictive body may be used instead of the piezoelectric body 22. The electrodes 23a and 23b are preferably made of metal, but may be made of a conductive material other than metal.
The protective matching layer 24 and the acoustic matching layer 25 efficiently propagate the ultrasonic vibration generated in the piezoelectric body 22 to the propagation medium 26, and efficiently transmit the ultrasonic wave propagated through the ultrasonic propagation medium 26 to the piezoelectric body 22. Has the ability to communicate.
The acoustic matching layer 25 of the present embodiment is preferably formed from a dry gel. The dry gel is a porous body formed by a sol-gel reaction, and is a material capable of extremely reducing acoustic impedance defined by the product of density ρ and sound velocity C (ρ × C). For this reason, by using the acoustic matching layer 25 formed from a dry gel, the transmission and reception efficiency of ultrasonic waves with respect to a gas such as air can be extremely increased.
A dry gel is obtained by forming a wet gel and then drying the wet gel. As the wet gel, first, a gel raw material solution is prepared, and a wet gel can be prepared by reaction of the gel raw material solution. The wet gel has a solid skeleton that is solidified by the reaction of the gel raw material solution, and the solid skeleton contains a solvent.
The dry gel obtained by drying the wet gel is a porous body and has continuous pores in the gaps of the solid skeleton part of about several nm to several μm. The average pore size is as small as 1 nm to several μm.
When the density of the dried gel is reduced by adjusting the production conditions, the sound velocity in the solid portion of the dried gel becomes extremely small, and the sound velocity in the gas portion in the pores becomes extremely small. Therefore, the sound speed of the dry gel shows a low value of 500 m / sec or less in a low density state, and shows a very low acoustic impedance. In particular, a dry gel having a solid skeleton portion and a pore size as small as several nanometers exhibits an extremely low sound velocity. In addition, since the pressure loss of gas is large in the nanometer-sized pores, when the acoustic matching layer is formed from the dried gel, sound waves can be emitted with a high sound pressure.
According to the manufacturing method described later, the acoustic impedance of the dried gel can be arbitrarily controlled within a wide range by adjusting the manufacturing process conditions even when the same raw material is used. Also, by changing the manufacturing process conditions, it is possible to produce an acoustic matching layer in which only the sound speed is changed while the density is substantially the same.
Dried gels have such advantageous characteristics but have low mechanical strength. For this reason, it was difficult to increase the manufacturing yield, and the reliability during use was also low. As described above with respect to the first to ninth embodiments, the production yield and reliability are improved by providing the member that protects the dried gel having low mechanical strength.
The protection unit in the first to ninth embodiments is extremely effective for improving the manufacturing yield of the ultrasonic transducer or the reliability in use, and can further control the thickness of the acoustic matching layer with high accuracy. This is effective for stabilizing the performance of the ultrasonic transducer. However, as described above, when a protective portion is provided on the surface (main surface) where the piezoelectric body emits or receives ultrasonic waves, the protective portion may be an acoustic obstacle. This is because, in the first to ninth embodiments, the thickness of the protective portion formed from a material different from the material of the acoustic matching layer is set to be substantially equal to the thickness of the acoustic matching layer. This is because the speed of sound is different between the protective portion and the acoustic matching layer, and the protective portion having the same thickness as the acoustic matching layer does not serve as the acoustic matching layer. For this reason, since the protection part of Embodiments 1-9 may become a factor which obstructs transmission / reception of an ultrasonic wave, it is preferable to arrange | position outside the main surface of a piezoelectric material.
However, there are cases where it is necessary to provide a protective part on the upper part of the piezoelectric body depending on ensuring reliability against more severe environmental conditions and limiting the outer diameter of the ultrasonic transducer.
In the present embodiment, a protection portion (formed from a material having a relatively high density and higher mechanical strength than the acoustic matching layer 25) is provided on the main surface of the piezoelectric body. A configuration that does not impair the performance as an ultrasonic transducer while having it is adopted.
In the present embodiment, the thickness of the protective portion provided on the main surface of the piezoelectric body 22 is set to about ¼ of the wavelength of the ultrasonic wave that is transmitted and received. Thereby, the protective part having a relatively high mechanical strength also functions as an acoustic matching layer. For this reason, in this specification, such a protection part may be referred to as a “protective matching layer”. By adopting such a configuration, the protection unit that protects the acoustic matching layer also plays a role as the acoustic matching layer, so that a highly sensitive ultrasonic transducer can be realized.
The thickness that best functions as an acoustic matching layer is 1/4 of the wavelength of the ultrasonic wave. On the other hand, the speed of sound in the protective matching layer 24 and the speed of sound in the acoustic matching layer 25 are different. Therefore, the thickness L3 of the protective matching layer 24 and the thickness L1 of the acoustic matching layer 25 have different sizes as shown in FIG. 20 (L3> L1).
If the thickness of the protective matching layer 24 and the acoustic matching layer 25 are both set to about 1/4 of the sound velocity, the thickness of the protective matching layer 24 is different from the thickness of the acoustic matching layer 25. In some cases, the ultrasonic wave emitted from the upper surface interferes with the ultrasonic wave emitted from the upper surface of the protective matching layer 24. In order to realize a highly sensitive ultrasonic transducer, the phase relationship between the ultrasonic waves radiated from each is extremely important.
22A shows an ultrasonic waveform on the upper surface of the protective matching layer 24, and FIG. 22B shows an ultrasonic waveform on the same level as the upper surface of the protective matching layer 24 above the acoustic matching layer 25. Is shown. Note that the symbol “ta” in FIG. 22B indicates the time during which the ultrasonic wave propagates through the ultrasonic wave propagation medium 26. One scale on the horizontal axis in each graph is about 3 μs when the ultrasonic frequency is 500 kHz.
The ultrasonic wave emitted from the upper surface of the acoustic matching layer 25 reaches the same level as the upper surface of the protective matching layer 24 through the ultrasonic propagation medium 26 such as a gas. For this reason, the phase relationship of the waveform of the ultrasonic wave at the same level as the upper surface of the protective matching layer 24 changes above the acoustic matching layer 25 depending on the speed of sound in the propagation medium 26 and the size L2 of the propagation medium 26.
Note that the signal waveforms in FIGS. 22A and 22B are obtained on the assumption that the wavelengths and amplitudes of the ultrasonic waves radiated from the protective matching layer 24 and the acoustic matching layer 25 are equal.
When the thickness L3 of the protective matching layer 24 and the thickness L1 of the acoustic matching layer 25 are each 1/4 of the ultrasonic wavelength in each layer, ultrasonic waves propagate between the lower surface and the upper surface of the protective matching layer 24. The time required for this is equal to the time required for the ultrasonic wave to propagate between the lower surface and the upper surface of the acoustic matching layer 25. Therefore, the phase of the ultrasonic wave that the ultrasonic wave radiated from the upper surface of the acoustic matching layer 25 reaches the same level as the upper surface of the protective matching layer 24 propagates through the protective matching layer 24 and reaches the upper surface of the protective matching layer 24. It is delayed compared to the phase of the reached ultrasonic wave. This phase delay corresponds to the time required for the ultrasonic wave emitted from the upper surface of the acoustic matching layer 25 to propagate through the propagation medium 26 by the distance L2.
The frequency of ultrasonic waves to be sent and received is f [seconds] ^-1 ], The time required for the ultrasonic wave to travel by a distance equal to one wavelength of the ultrasonic wave is 1 / f [second]. The time t3 required for the ultrasonic wave to pass through the protective matching layer 24 of this embodiment is ¼ f [second]. On the other hand, the time t2 required for the ultrasonic wave to pass through the acoustic matching layer 25 of the present embodiment is also 1/4 f [second]. Here, if the time required for the ultrasonic wave to propagate through the propagation medium 26 by the distance L2 is t2 (= ta), the ultrasonic wave radiated from the upper surface of the protective matching layer 24 depends on the time t2. Interference occurs between the sound wave and the ultrasonic wave emitted from the upper surface of the acoustic matching layer 25. This interference changes the ultrasonic waveform and sensitivity.
FIG. 22C shows an ultrasonic waveform observed when the time t2 is 1 / 2f [second], and FIG. 22D shows a case where the time t2 is 1 / f [second]. Shows the observed ultrasonic waveform. As can be seen from FIGS. 22C and 22D, the sensitivity of the observed ultrasonic wave varies greatly depending on the value of time t2. When the time t2 is equal to 1/2 f [second], the phase shift becomes the half wavelength of the ultrasonic wave, and the sensitivity of the observed ultrasonic wave is low. On the other hand, when the time t2 is 1 / f [second], the phase shift is an integral multiple of the wavelength of the ultrasonic transducer, so that the sensitivity of the observed ultrasonic wave is high. When the time t2 is in the range of 1 / 2f to 1 / f [second], the transmission / reception sensitivity of the ultrasonic wave increases as t2 approaches 1 / f [second] from 1 / 2f [second].
When the ultrasonic wave radiated from the acoustic matching layer 25 propagates through the propagation medium 26 and reaches the same level as the upper surface of the protective matching layer 24, the phase of the ultrasonic wave propagates through the protective matching layer 24. By adjusting the thicknesses of the acoustic matching layer 25 and the protective matching layer 24 so as to substantially coincide with each other, a highly sensitive ultrasonic transducer can be provided. In the present specification, when “the phases are substantially matched”, it means that the difference in phase of the ultrasonic wave is ¼ or less of the ultrasonic wavelength, and the smaller the phase difference is, the better.
FIG. 23 is a cross-sectional view schematically showing the phase of an ultrasonic wave when the time t2 is 1 / f [second]. In this figure, the phase of the ultrasonic wave on the upper surface of the protective matching layer 24 and the phase of the ultrasonic wave above the acoustic matching layer 25 and at the same level as the upper surface of the protective matching layer are the same. When such phase matching occurs, the ultrasonic wave transmission / reception sensitivity is maximized. Even when such complete phase matching does not occur, if the phase shift is set to be small, the transmission / reception sensitivity of the ultrasonic wave is sufficiently improved as compared with the prior art. The phase shift is preferably adjusted to 1/4 or less of the ultrasonic wavelength in the ultrasonic propagation medium, and more preferably adjusted to 1/8 or less.
By simply controlling the thickness L1 of the acoustic matching layer 25 and the thickness L3 of the protective matching layer 24 to about 1/4 of the ultrasonic wavelength in the acoustic matching layer 25 and the protective matching layer 24, respectively, the magnitude of L2 is Since it is uniquely determined as (L3-L1), t2 cannot be set arbitrarily. Therefore, in order to set the time t2 to a desired magnitude, not only the thickness of the acoustic matching layer 25 and the protective matching layer 24 but also the sound velocity in the acoustic matching layer 25 and the protective matching layer 24 is appropriately controlled. There is a need. In a preferred embodiment of the present invention, the acoustic matching layer 25 is formed from a dry gel whose sound speed can be easily controlled.
Next, an embodiment of a method for manufacturing the ultrasonic transducer 21 of the present embodiment will be described with reference to FIGS. In the present embodiment, the ultrasonic propagation medium 26 is air (density: 1.18 kg / m ³ , Sound speed: about 340 m / s, acoustic impedance about 4.0 × 10 ² kg / m ² / S).
First, as shown in FIG. 24A, a piezoelectric body 22 is prepared according to the wavelength of ultrasonic waves to be transmitted and received. The piezoelectric body 22 at this stage is not provided with the protective matching layer 24 shown in FIG. The piezoelectric body 22 is preferably made of a highly piezoelectric material such as piezoelectric ceramics or a piezoelectric single crystal. As the piezoelectric ceramic, lead zirconate titanate, barium titanate, lead titanate, lead niobate, or the like can be used. As the piezoelectric single crystal, lead zirconate titanate single crystal, lithium niobate, quartz, or the like can be used.
In the present embodiment, lead zirconate titanate ceramics are used as the piezoelectric body 22, and the wavelength of ultrasonic waves to be transmitted and received is set to 500 kHz. In order to allow the piezoelectric body 22 to efficiently transmit and receive such ultrasonic waves, the resonance frequency of the piezoelectric body 22 is designed to be 500 kHz. For this reason, in this embodiment, the piezoelectric body 22 formed from a piezoelectric ceramic having a cylindrical shape with a diameter of 12 mm and a thickness of about 3.8 mm is used. Electrodes 23a and 23b are formed on both surfaces of the piezoelectric body 22 by baking of silver, and polarization treatment is performed in this direction.
Next, three ring-shaped members functioning as the protective matching layer 24 are prepared and bonded to the main surface of the piezoelectric body 22 as shown in FIG. At this time, the centers of the ring-shaped members are aligned with the centers of the piezoelectric bodies 22 as shown in FIG. The three ring-shaped members functioning as the protective matching layer 24 are a first ring-shaped member having an outer diameter of 12 mm, an inner diameter of 11 mm, and a thickness of 1.0 mm, an outer diameter of 8 mm, an inner diameter of 7 mm, and a thickness of 1.0 mm, respectively. A ring-shaped member and a third ring-shaped member having an outer diameter of 4 mm, an inner diameter of 3 mm, and a thickness of 1.0 mm.
The protective matching layer 24 in this embodiment is not only required to have a high mechanical strength and a function capable of protecting the acoustic matching layer, but also to have a relatively low acoustic impedance in order to fulfill the function of the acoustic matching layer. It is done. In this embodiment, a porous ceramic is used as such a material. This porous ceramic has an apparent density of 0.64 × 10 6. ³ kg / m ³ , Sound speed is 2000m / s, acoustic impedance is about 1.28 × 10 ⁶ kg / m ³ It is. As the ceramic, a barium titanate-based material is used. The “apparent density” is a density including a space part included in the porous body. About 80% of the volume of the porous ceramic is a space portion, and the substantial portion of the ceramic is about 20% by volume.
As described above, since the sound velocity of the protective matching layer 24 is about 2000 m / s, the quarter thickness of the wavelength at 500 kHz corresponds to 1.0 mm. For this reason, in this embodiment, the thickness of the ring-shaped member that functions as the protective matching layer 24 is set to 1.0 mm.
The porous ceramic used in the present embodiment can be produced as follows.
First, resin-made fine balls and ceramic powder are mixed and pressed. Thereafter, the ceramic is sintered. In this sintering process, the resin balls are heated, burned and removed. When heating is performed rapidly during sintering, the resin balls may expand or rapidly gasify, and the ceramic structure may be destroyed. For this reason, the sintering is preferably performed by gentle heating.
In the present embodiment, the protective matching layer 24 formed from such porous ceramics and the piezoelectric body 22 are joined by bonding with an adhesive. For example, an epoxy resin is used as an adhesive, and when left in a thermostatic bath at 150 ° C. for about 2 hours while applying a pressure of about 0.1 MPa, the adhesive is cured, and the protective matching layer 24 and the piezoelectric body 22 Join.
Next, an acoustic matching layer 25 is provided on the composite body formed of the piezoelectric body 22 / protective matching layer 24 as shown in FIG. In the present embodiment, the acoustic matching layer 25 is formed from a dry gel.
In the present embodiment, first, the acoustic matching layer 25 having the thickness shown in FIG. 24B is formed, and then the acoustic matching layer 25 is thinned as shown in FIG. At this time, the thickness L3 of the protective matching layer 24 and the thickness L1 of the acoustic matching layer are set so that the distance L2 (= L3-L1) shown in FIG. 20 is equal to one wavelength of the ultrasonic wave in the air. Specifically, since the frequency of ultrasonic waves to be transmitted and received is 500 kH, one wavelength of the ultrasonic waves in the air is 0.62 mm. On the other hand, since the thickness L3 of the protective matching layer 24 is 1.0 mm, the thickness L1 of the acoustic matching layer 25 is 0.32 mm (= 1.0 mm−0.62 mm). In order for the acoustic matching layer 25 to function properly as the acoustic matching layer, the thickness L1 (= 0.32 mm) is most preferably ¼ of the wavelength of the ultrasonic wave propagating through the acoustic matching layer 25. desirable. Therefore, it is necessary to have a material characteristic having a sound speed such that 0.32 mm is 1/4 of the wavelength of the ultrasonic wave transmitted and received. According to the calculation, the acoustic matching layer 25 having a thickness of 0.32 mm may be formed from a dry gel having a sound speed of 640 m / s.
The thickness of the protective matching layer 24 is preferably ¼ of the ultrasonic wavelength in the protective matching layer 24, but is not limited to that size. It may be in the range of 1/8 to 1/3 of the ultrasonic wavelength, and more preferably in the range of 1/6 to 1/4 of the ultrasonic wavelength. When there is a distribution in the wavelength of the ultrasonic wave, it is preferable to determine the thickness based on the peak wavelength. In this specification, when the wavelength is distributed, “¼ of the wavelength” means “¼ of the peak wavelength”.
When the acoustic matching layer 25 is a single layer, the thickness of the acoustic matching layer 25 is also preferably ¼ of the ultrasonic wavelength in the acoustic matching layer 25, but is not limited to that size. It may be in the range of 1/8 to 1/3 of the ultrasonic wavelength, and more preferably in the range of 1/6 to 1/4 of the ultrasonic wavelength. When the acoustic matching layer has a multilayer structure, it is preferable that each constituent layer has the above thickness. An ultrasonic transducer in which the acoustic matching layer has a multilayer structure will be described later as a second embodiment.
As a material of the dry gel constituting the acoustic matching layer 25, various materials such as an inorganic material and an organic polymer material can be used. As the solid skeleton portion of the inorganic material, silicon oxide (silica), aluminum oxide (alumina), titanium oxide, or the like can be used. Further, as the solid skeleton portion of the organic material, a general thermosetting resin or thermoplastic resin can be used. For example, polyurethane, polyurea, phenol resin, polyacrylamide, polymethyl methacrylate, or the like can be used.
In the present embodiment, as a material of the acoustic matching layer 25, silicon oxide (silica) is used as a solid skeleton portion from the viewpoint of cost, environmental stability, ease of manufacturing, stable temperature characteristics of the ultrasonic transducer, and the like. Adopt dry gel.
The sound speed of 640 m / s is a relatively high value as the sound speed of the dry gel. For this reason, in this embodiment, when a dry gel layer is formed as the acoustic matching layer 25, a conventional manufacturing method in which a drying step is performed subsequent to a gelation step (hereinafter referred to as "first gelation step"). Instead, a method of performing the second gelation step after the first gelation step is adopted.
When only the first gelation step is performed without performing the second gelation step, it is difficult to obtain a dry gel showing a relatively high sound speed. In addition, since the density of the dried gel increases in proportion to the sound speed, “high sound speed” means “high density”. If the concentration of the gel raw material in the gel raw material liquid is increased for the purpose of increasing the sound speed of the gel, the gelation reaction does not proceed uniformly, and a wet gel having a random sound speed distribution is formed. A dry gel obtained by drying the wet gel also has a random density distribution. For this reason, when the concentration of the gel raw material in the gel raw material liquid is increased, it becomes extremely difficult to make the sound speed uniform.
In this embodiment, in order to avoid gel non-uniformity, the speed of sound of the dried gel formed in the first gelation step is adjusted to about 200 m / s or less, and the density is further increased by the second gelation step. Increase the speed of sound. In the second gelation step, the wet gel obtained in the first gelation step is again immersed in the gel raw material liquid (second gelation raw material liquid). In the second gelation step, the concentration of ammonia serving as a catalyst in the second gelation raw material liquid is adjusted to be low. For this reason, gelation does not occur outside the wet gel obtained in the first gelation step. However, inside the wet gel obtained in the first gelation step, the second gelation raw material liquid grows so as to adhere to the skeleton formed in the first gelation step. For this reason, this reaction proceeds even under conditions where the gel raw material liquid itself does not gel. In this way, the sound speed and density of the gel can be changed.
Specifically, the acoustic matching layer 25 made of dry gel is formed by performing the following steps.
Step 1: Preparation of first gelled gel raw material liquid
Tetraethoxysilane / ethanol / water / hydrogen chloride are mixed at a molar ratio of 1/2/1 / 0.00078, and the hydrolysis of tetraethoxysilane is allowed to proceed for 3 hours in a thermostatic bath at 65 degrees. In addition, water / NH ₃ Is prepared by adding a ratio of 2.5 / 0.0057 (molar ratio to tetraethoxysilane) and mixing them.
Process 2: First gelation process
The gel raw material liquid (first gelation raw material liquid) adjusted as described above is dropped into the space formed by the piezoelectric body 22 and the protective matching layer 24. At this time, a Teflon sheet is wound around the outer periphery of the outermost protective matching layer 24 to form a frame so that the gel raw material liquid does not spill.
The sample to which the gel raw material liquid has been dropped is left at 50 ° C. for about 1 day while keeping the level in a thermostatic bath. Thus, the gel raw material liquid supplied in the space formed by the piezoelectric body 22 and the protective matching layer 24 gels to form a wet gel.
Step 3: Second gelation step (adjustment of sound speed and density)
When the acoustic matching layer obtained in the first gelation step is directly subjected to the drying step, the density is 2.0 × 10. ² kg / m ³ The sound speed is about 200 m / s. In the present embodiment, the second gelation step is performed for the purpose of further increasing the sound speed and density.
First, the wet gel obtained in the first gelation step is washed with ethanol to prepare a second gelation raw material liquid. As the second gelation raw material liquid, a mixture of tetraethoxysilane / ethanol / 0.1N aqueous ammonia and 60/35/5 in volume ratio is used.
The composite composed of the piezoelectric material 22 / wet gel / protective matching layer 24 obtained in the first gelation step is immersed in the second gelation raw material liquid in a sealed container and left in a constant temperature bath at 70 ° C. for about 48 hours. To do. By this second gelation step, the gel skeleton obtained in the first gelation step grows, and the density and sound speed increase.
Process 4: Hydrophobization process
The hydrophobizing step is not necessarily required, but it is preferable to perform the hydrophobizing step because the performance may deteriorate due to moisture absorption. In the hydrophobization step, after the second gelation step, the second gelation raw material liquid remaining in the wet gel is replaced and washed with ethanol, and then dimethyldimethoxysilane / ethanol / 10 wt% ammonia water is added. The hydrophobizing step is performed by immersing in a hydrophobizing solution obtained by mixing at a weight ratio of 45/45/10 at 40 ° C. for about 1 day.
Process 5: Drying process
In order to obtain a dry gel from the wet gel obtained in the above steps, a drying step is performed. In this embodiment, a supercritical drying method is used as the drying method. As mentioned above, a dry gel is a very small nanometer-sized porous material. Depending on the thickness of the skeleton, the strength of the bond, and the size of the pores, solvent drying from the wet gel to the dry gel is possible. In this case, it may be destroyed by the surface tension of the solvent.
For this reason, the supercritical drying method in which surface tension does not work can be used effectively. Specifically, after replacing the above hydrophobized liquid with ethanol, the composite of piezoelectric body 22 / wet gel / protective part polishing acoustic matching layer 25 obtained in the above steps is put in a pressure resistant container, and the wet gel is added. The ethanol inside is replaced with liquefied carbon dioxide.
Furthermore, the pressure in the container is increased to 10 MPa by sending liquefied carbon dioxide into the container with a pump. Then, the inside of a container was made into the supercritical state by heating up to 50 degreeC. Next, drying is completed by slowly releasing the pressure while maintaining the temperature at 50 ° C.
Process 6: Thickness adjustment process
The dried gel layer thus formed was ground on the acoustic matching layer 25 only by a lathe so that the thickness was 0.32 mm.
The density of the dried gel forming the acoustic matching layer 25 thus obtained is about 0.6 × 10 ³ kg / m ³ The sound speed is about 640 m / s. Moreover, although the dry gel used as the acoustic matching layer 25 has permeated into a part of the protective matching layer 24, the sound velocity of the protective matching layer 24 is not affected.
Before the step of forming the acoustic matching layer from the dried gel, it is preferable to treat the surface of the electrode 23b so that the adhesion between the electrode 23b and the acoustic matching layer 25 is improved. When the adhesion between the electrode 23b and the acoustic matching layer 25 is increased by the surface treatment, the reliability is further improved. As such a surface treatment, a plasma treatment or the like in which a hydroxyl group is imparted to an electrode on the surface of a piezoelectric body that is easily chemically bonded to a dry gel can be employed. Alternatively, it is also effective to impart an anchor effect by forming physical irregularities on the surface of the electrode 23b. Specifically, a chemical and / or physical etching process can be suitably employed.
In this embodiment, after forming the gel used as the acoustic matching layer 25, it grinds with a lathe and adjusts the thickness of a dry gel. The adjustment of the thickness may be performed by adjusting the amount (height) of the first gelation raw material liquid dropped during the first gelation step. In this case, 33.9 μL of the gel raw material liquid is accurately weighed with a micropipette and dropped onto the piezoelectric body 22 so that the thickness of the acoustic matching layer to be formed is finally about 0.32 mm. . Since the protective matching layer 24 is a porous body having 80% voids, it is necessary to calculate the dripping amount by converting the volume absorbed by the porous body.
The transmission / reception waveform of the ultrasonic transducer thus manufactured is shown in FIG. In FIG. 25, the ultrasonic transmission / reception waveform of the present embodiment is indicated by a solid line, and the ultrasonic transmission / reception waveform of an ultrasonic transmitter / receiver (comparative example) in which the protective portion and the acoustic matching layer 25 have the same thickness is indicated by a dotted line. Has been. As can be seen from FIG. 25, according to this embodiment, the amplitude of the signal increases.
High sensitivity can be achieved by using the structure of the present invention.
In the present embodiment, in order to align the phase of the ultrasonic wave at the same level as the upper surface of the protective matching layer 24, the ultrasonic wave that has propagated through the acoustic matching layer 25 and the propagation medium 26 has propagated through the protective matching layer 24. Compared to sound waves, the phase is delayed by exactly the wavelength. When the protective matching layer 24 is formed from a material having a higher sound velocity, or when the thickness L3 of the protective matching layer 24 is set to be large, the phase delay due to the propagation medium 26 is set to two or more ultrasonic wavelengths. May be.
(Embodiment 11)
Next, an eleventh embodiment of the ultrasonic transducer of the present invention will be described with reference to FIG. The main feature of this embodiment is that the acoustic matching layer has a laminated structure including a lower first acoustic matching layer 25a and an upper second acoustic matching layer 25b.
Even when the acoustic matching layer 25 has a two-layer structure, the thickness of each acoustic matching layer 25a, 25b is preferably set to about 1/4 of the ultrasonic wavelength in each acoustic matching layer.
Also in the present embodiment, as in the tenth embodiment, in order to align the phase of the ultrasonic wave at the same level as the upper surface of the protective matching layer 24, the ultrasonic wave propagated through the acoustic matching layer 25 and the propagation medium 26 is Compared to the ultrasonic wave propagating through the protective matching layer 24, a phase delay corresponding to substantially an integral multiple of the ultrasonic wavelength is generated.
In the configuration shown in FIG. 26, when an ultrasonic wave propagates through the protective matching layer 24 and reaches the upper surface of the protective matching layer 24, the ultrasonic wave having the same phase as that of the ultrasonic wave has a first acoustic matching layer and a second acoustic matching layer. The boundary surface of 25b is reached. This is because the sound speed of the first acoustic matching layer 25 a is smaller than the sound speed of the protective matching layer 24. It takes an additional ¼ f [second] for the ultrasonic wave to reach the upper surface of the second acoustic matching layer 25b from the upper surface of the first acoustic matching layer 25a. Therefore, if the time until the propagation medium 26 propagates from the upper surface of the acoustic matching layer 25b to reach the same level as the upper surface of the protective matching layer 24 is set to 3 / 4f [seconds], the upper surface of the protective matching layer 24 is set. The phase is aligned at the level. When such a configuration is adopted, a shift of one wavelength occurs between the ultrasonic wave transmitted through the acoustic matching layers 25a and 25b and the ultrasonic wave transmitted through the protective matching layer 24. Since both ultrasonic waves interfere and strengthen each other, the amplitude of the ultrasonic waves increases.
A method for manufacturing the acoustic matching layers 25a and 25b in the present embodiment will be described.
First, the protective matching layer 24 is produced in the same manner as the manufacturing method of the acoustic matching layer 25 in the tenth embodiment. Porous ceramics is used as the material of the protective matching layer 24, and its thickness (L7) is set to 1.0 mm.
In the present embodiment, the distance (L6) from the upper surface of the second acoustic matching layer 25b to the upper surface level of the protective matching layer 24 is set so that the time for propagating in the propagation medium 26 such as air is 3 / 4f [seconds]. Set to 0.51 mm. As a result, the total thickness (L4 + L5) of the first acoustic matching layer 25a and the second acoustic matching layer 25b is equal to 0.49 mm.
In the present embodiment, when the sound speed of the second acoustic matching layer 25b is set to 200 m / s, the thickness (L5) of the second acoustic matching layer 25b is preferably set to 0.10 mm. When L5 = 0.10 mm, the thickness (L4) of the first acoustic matching layer 25a is 0.39 mm (= 0.49 mm−0.10 mm). In order for the thickness of the first acoustic matching layer 25a to correspond to a quarter wavelength of the ultrasonic wave in the first acoustic matching layer 25a, the sound speed of the first acoustic matching layer 25a needs to be 780 m / s. .
Next, a manufacturing method of the two acoustic matching layers 25a and 25b will be described. A characteristic point of this method is that the second gelation step performed in the tenth embodiment is performed twice. That is, in this embodiment, after performing the 2nd gelation process (2-1 gelation process) which does not gel on the outside of the wet gel formed at the 1st gelation process, it is gel also on the outside of the wet gel. A second gelation step (second-2 gelation step) in which crystallization occurs is performed.
In the present embodiment, first, the first acoustic matching layer 25a is formed by performing the same processes as the processes 1 to 6 performed in the tenth embodiment. However, the second gelation step at this time is a 2-1 gelation step.
In view of an increase in acoustic impedance in the 2-2 gelling step performed thereafter, in the 2-1 gelling step, the density of the first acoustic matching layer 25a is about 0.5 × 10. ³ kg / m ³ The processing time is adjusted so that the sound speed is about 500 m / s. In the present embodiment, the processing time is set to about 36 hours, which is shorter than the processing time of the second gelation step in the tenth embodiment.
Next, by performing the 2-2 gelling step, the acoustic impedance of the first acoustic matching layer 25a is increased, and the second acoustic matching layer 25b is formed on the first acoustic matching layer 25a. Specifically, the 2-2 gelation step was performed as follows.
2-2 Gelation step:
First, a liquid in which tetraethoxysilane / ethanol / 0.05N aqueous ammonia is mixed at a molar ratio of 1/4/3 is prepared as the 2-2 gelling raw material liquid. This 2-2 gelling raw material liquid is filled in the space formed by the first acoustic matching layer 25a and the protective matching layer 24. Next, the gelation is completed by leaving it at room temperature for about 24 hours. Thus, the acoustic impedance of the first acoustic matching layer 25a is adjusted, and a wet gel that becomes the second acoustic matching layer 25b is formed.
Thereafter, the acoustic matching layers 25a and 25b are completed by performing the hydrophobizing step, the drying step, and the thickness adjusting step in the same manner as in the tenth embodiment. The acoustic matching layers 25a and 25b in the present embodiment are characterized as follows.
First acoustic matching layer 25a
Density: 0.7 × 10 ³ kg / m ³ , Sound speed: 780m / s
Acoustic impedance: 5.46 × 10 ⁵ kg / m ² / S
Thickness: 0.39mm
Second acoustic matching layer 25b
Density: 0.2 × 10 ³ kg / m ³ , Sound speed: 200m / s
Acoustic impedance: 4.0 × 10 ⁴ kg / m ² / S
Thickness: 0.10mm
The transmission / reception waveform of the ultrasonic transceiver according to the present embodiment is shown in FIG. In FIG. 27, the ultrasonic transmission / reception waveform of the ultrasonic transmission / reception unit of this embodiment is indicated by a solid line, and the transmission / reception waveform form of the ultrasonic transmission / reception unit (comparative example) in which the thicknesses of the acoustic matching layer and the protective matching layer are equal. Is indicated by a dotted line. As can be seen from FIG. 27, according to the ultrasonic transducer of this embodiment, high sensitivity can be realized.
In the present embodiment, the acoustic matching layer 25 is composed of two layers. However, even if three or more layers are used, the same effect can be obtained by designing the upper surface portion of the protective matching layer 24 so that the phases of the ultrasonic waves are aligned. can get.
Embodiment 12
A twelfth embodiment of the ultrasonic transducer according to the present invention will be described with reference to FIG. The characteristic point of this embodiment is that the first acoustic matching layer 25a and the protective matching layer 24 are integrally formed of the same material. A second acoustic matching layer 25b made of dry gel is formed on the first acoustic matching layer 25a.
In the present embodiment, the sound velocity and wavelength of the ultrasonic waves in the first acoustic matching layer 25a and the protective matching layer 24 are equal, and the thickness L11 of the protective matching layer 24 is set to ¼ wavelength of the ultrasonic waves. For this reason, the thickness L8 of the first acoustic matching layer 25a is smaller than the 1/4 wavelength of the ultrasonic wave. The thickness L8 of the first acoustic matching layer 25a is determined by the thickness L9 of the second acoustic matching layer 25b and the distance L10 from the upper surface of the second acoustic matching layer 25b to the upper surface level of the protective matching layer 24.
Also in the present embodiment, a phase delay of an integral multiple of the ultrasonic wavelength between the ultrasonic wave radiated through the acoustic matching layers 25a and 5b and the ultrasonic wave radiated through the protective matching layer 24 is obtained. Adopting a configuration that causes For this reason, at the upper surface level of the protective matching layer 24, the phases of the ultrasonic waves propagating through the acoustic matching layers 25a and 25b and the propagation medium 26 are aligned.
In order to increase the sensitivity of the ultrasonic waves transmitted through the acoustic matching layers 25a and 25b, the thickness of the second acoustic matching layer 25b is more important than the thickness of the first acoustic matching layer 25a. In the present embodiment, the thickness of the second acoustic matching layer 25b is set to about ¼ of the wavelength of ultrasonic waves to be transmitted and received. The thickness of the first acoustic matching layer 25a also affects the sensitivity, but most of the influence extends to the frequency ratio band.
For this reason, in this embodiment, the characteristic of the dry gel layer which can comprise the 2nd acoustic matching layer 25b is first determined from points, such as mechanical strength of a material. Next, the thickness L8 of the first acoustic matching layer 25a formed from the same material as the protective matching layer 24 and the thickness L10 of the acoustic wave propagation medium are set.
In the present embodiment, the porous matching ceramic is used as the material of the protective matching layer 24 as in the above-described embodiment, and the thickness (L11) is set to ¼ of the ultrasonic wavelength. That is, L11 is set to 1.0 mm. In this case, the sound velocity of the first acoustic matching layer 25b formed from the porous ceramic is 200 m / s, and the density is 0.2 × 10. ³ kg / m ³ It becomes. In order to set the thickness of the second acoustic matching layer 25b formed from the dried gel to ¼ of the ultrasonic wavelength, L9 is set to 0.10 mm.
At this time, the following Expression 6 is established.

Equation 6 is derived from setting L11 to 1.0 mm and L9 to 0.1 mm.
In order to exhibit excellent characteristics, it is preferable that the following expression holds.

In the present embodiment, since ultrasonic waves having a frequency of 500 kHz are transmitted and received, one wavelength of the ultrasonic waves in the acoustic matching layer 25a is 1.0 mm, and one wavelength of the ultrasonic waves in the sound wave propagation medium 26 is 17/25 mm. Equation 7 is the sum of the ratio of the thickness L8 of the first acoustic matching layer 25a to one wavelength of the ultrasonic wave and the ratio of the thickness L10 of the propagation medium 26 to one wavelength of the ultrasonic wave. Satisfying Equation 8 means that the ultrasonic wave travels by one wavelength when passing through the first acoustic matching layer 25a and the sound wave propagation medium 26. In other words, it means that the effective thickness of the first acoustic matching layer 25a and the sound wave propagation medium 26 felt by the ultrasonic waves is one wavelength.
When L8 and L10 satisfying Expression 6 and Expression 7 are calculated, L8 = about 0.69 mm and L10 = 0.21 mm.
Next, a method for manufacturing the ultrasonic transducer according to this embodiment will be described with reference to FIGS.
First, as shown in FIG. 29A, a 1.0 mm thick pellet made of porous ceramics is prepared, and this pellet is processed as shown in FIG. 29B. In this embodiment, a groove is formed on the upper surface of the pellet, and the thickness of the groove bottom is adjusted to 0.69 mm. This groove bottom is a portion that functions as the first acoustic matching layer 25a. The groove is formed in a ring shape as shown in FIG.
Next, as shown in FIG. 29C, the second acoustic matching layer 25b is formed inside the groove. The thickness of the acoustic matching layer 25b is set to 0.1 mm. The composite of the protective matching layer 24 / acoustic matching layers 25a and 25b is joined to the piezoelectric body 22 as shown in FIG. 29 (d) to form the ultrasonic transducer 21.
The first acoustic matching layer 25b is a first gel using a liquid obtained by mixing tetramethoxysilane / ethanol / 0.05N aqueous ammonia at a molar ratio of 1/7/4 in the same manner as in the first embodiment. It is formed by performing a crystallization process.
Adhesion of the composite composed of the protective matching layer 24 / acoustic matching layers 25a and 25b to the piezoelectric body can be performed with an epoxy adhesive, as in the first embodiment.
According to this embodiment, since the protective matching layer 24 and the acoustic matching layer 25a can be collectively formed, the manufacturing process can be simplified and the manufacturing cost can be reduced.
(Embodiment 13)
A thirteenth embodiment of an ultrasonic transducer according to the present invention will be described with reference to FIG. A characteristic point of this embodiment is that it has a structural support.
The ultrasonic transducer according to the present embodiment is the ultrasonic wave according to the first embodiment except that the structural support 7 is provided between the piezoelectric body 22 and the first acoustic matching layer 25 or the protective matching layer 24. It has the same configuration as that of the transmission / reception recording medium.
The structural support 7 includes a disk-shaped support portion to which the acoustic matching layer 25 and the like are fixed, and a cylindrical portion that extends continuously from the disk-shaped support portion in the axial direction. The end surface of the cylindrical portion is bent in an L shape and is easily fixed to a plate (not shown) for shielding the piezoelectric body 22 or another device.
An acoustic matching layer 25 and a protective matching layer 24 are disposed on the surface of the structural support 7, and a piezoelectric body 22 is disposed on the back surface of the support portion. By using such a structural support 13, the ultrasonic transducer can be handled very easily.
The structural support can be composed of a container (sensor case) that can be sealed. In this case, if the open end of the cylindrical portion of the structural support 27 is closed with a shielding plate or the like, and the inside of the structural support 27 is filled with an inert gas, the piezoelectric body 22 is blocked from the fluid whose flow rate is to be measured. be able to.
Since a voltage is applied to the piezoelectric body 22, there is a risk that the combustible gas may be ignited when the piezoelectric body comes into contact with the combustible gas. However, the structural support 27 is formed of a hermetically sealed container, and the inside of the piezoelectric body 22 is cut off from an external fluid or the like to prevent such ignition and to be safe against flammable gases. Sound waves can be transmitted and received.
Even when not a combustible gas, the piezoelectric body 22 is shielded from external gas even when ultrasonic waves are transmitted to and received from a gas that reacts with the piezoelectric body 22 and may deteriorate the characteristics of the piezoelectric body 22. It is preferable to do. By doing so, it is possible to prevent the piezoelectric body 22 from deteriorating and realize a highly reliable operation over a long period of time.
Of the structural support 27, the portion located between the piezoelectric body 22 and the acoustic matching layer 25 or the protective matching layer 24 does not function as an acoustic matching layer. Therefore, in order to prevent the structural support 27 from acting as an acoustic obstacle, the thickness of the portion of the structural support 27 that is located between the piezoelectric body 22 and the acoustic matching layer 25 or the protective matching layer 24. Is preferably about 1/8 or less of the wavelength of ultrasonic waves to be transmitted and received.
In the present embodiment, the structure support 27 is made of stainless steel, and the thickness of the portion is set to 0.2 mm.
The speed of sound of stainless steel is about 5500 m / second, and the wavelength of ultrasonic waves at 500 kHz is about 11 mm. Since the thickness of 0.2 mm corresponds to about 1/55 of the wavelength, the presence of the structural support 7 is hardly an acoustic impediment factor.
The material of the structural support 27 is not limited to a metal material such as stainless steel, and a material corresponding to the purpose is selected from ceramic, glass, resin, and the like. In this embodiment, the external fluid and the piezoelectric body are reliably separated, and even if some mechanical impact is applied to the structural support body, the metal material is provided with a strength capable of preventing contact between the piezoelectric body and the external fluid. The structural support 27 is manufactured from the above. Thereby, for example, high safety can be ensured even when ultrasonic waves are transmitted and received for flammable or explosive gases.
In addition, when performing ultrasonic transmission / reception with respect to safe gas, you may use the structure support body which consists of materials, such as resin, for the purpose of cost reduction.
(Embodiment 14)
A fourteenth embodiment of an ultrasonic transducer according to the present invention will be described with reference to FIGS. 31 (a) and (b). FIGS. 31A and 31B are top views of the present embodiment.
In the example shown in FIG. 21, a porous ceramic ring (three ring-shaped members having the same width and different diameters) functioning as the protective matching layer 24 is used and arranged on the main surface of the piezoelectric body so that their centers coincide. However, the protective matching layer 24 may be formed using rings having different widths as shown in FIG. Further, as shown in FIG. 31B, the island-shaped protective matching layers 24 may be randomly arranged.
When the protective matching layer 24 and the acoustic matching layer 25 are regularly arranged on the main surface of the ultrasonic transducer, the phase of the ultrasonic wave is in a direction having a certain angle with respect to the main surface. As a result, the amplitude increases. This is called a “sidelobe” and becomes an obstruction factor when performing ultrasonic measurement. However, as shown in FIG. 31, by adopting a configuration in which the arrangement of the protective matching layer 24 does not have periodicity, it is possible to suppress side lobes and enable ultrasonic measurement with high accuracy and reliability. .
(Embodiment 15)
A fifteenth embodiment of an ultrasonic transducer according to the present invention will be described with reference to FIG.
The ultrasonic transducer according to this embodiment has a first characteristic point in that the thickness of the protective matching layer 24 has an in-plane distribution. In each of the above-described embodiments, the thickness of the protective matching layer 24 is set uniformly in the plane, but in this embodiment, an in-plane distribution is intentionally given. The second feature of the present embodiment is that the protective matching layer 24 provided on the piezoelectric body 22 is formed of two different materials.
According to the configuration of the present embodiment, by using different materials and / or providing in-plane distributions of different thicknesses, side lobes can be suppressed, or the frequency of ultrasonic waves to be transmitted and received can be changed to broaden the bandwidth. Can do.
The thickness of each protective matching layer 24 is preferably included in the range of 1/8 to 1/3 of the ultrasonic wavelength, and in the range of 1/6 to 1/4 of the ultrasonic wavelength. More preferably, it is contained. However, the thickness of a part of the protective matching layer 24 having a different thickness may be out of the above range. Since the protective matching layer 24 having a thickness outside the above range does not function as an acoustic matching layer, the sensitivity of ultrasonic transmission / reception decreases. However, by placing a protective layer that does not function as an acoustic matching layer (which can no longer be called a “protective matching layer”) at an appropriate position on the piezoelectric body, the disturbance of the ultrasonic field at a short distance can be prevented and a good super Sound wave measurement can be made possible.
(Embodiment 16)
An embodiment of an ultrasonic flowmeter according to the present invention will be described with reference to FIG.
The ultrasonic flowmeter of the present embodiment is installed so that the fluid to be measured flows at a velocity V in a pipe that functions as the flow rate measurement unit 51. On the tube wall 52 of the flow rate measuring unit 51, ultrasonic transmission receivers 1a and 1b formed from the ultrasonic transmitter / receiver of the present invention are arranged to face each other.
At some point, the ultrasonic transmitter / receiver 1a functions as an ultrasonic transmitter and the ultrasonic transmitter / receiver 1b functions as an ultrasonic receiver. At other points, the ultrasonic transmitter / receiver 1a operates as an ultrasonic receiver. The ultrasonic transmitter / receiver 1b functions as an ultrasonic transmitter. This switching is performed by the switching circuit 53.
The ultrasonic transceivers 1a and 1b are connected via a switching circuit 53 to a drive circuit 54 that drives the ultrasonic transceivers 1a and 1b and a reception detection circuit 55 that detects ultrasonic pulses. The output of the reception detection circuit 55 is sent to a timer 56 that measures the propagation time of the ultrasonic pulse. The output of the timer 56 is sent to a calculation unit 57 that calculates the flow rate. The computing unit 57 calculates the velocity V of the fluid flowing in the flow rate measuring unit 51 based on the measured propagation time of the ultrasonic pulse, and obtains the flow rate. The drive circuit 54 and the timer 56 are connected to the control unit 58 and controlled by a control signal output from the control unit 58.
Hereinafter, the operation of this ultrasonic flowmeter will be described in more detail.
Consider a case where LP gas flows through the flow rate measuring unit 51 as a fluid to be measured. The drive frequency of the ultrasonic transmission receivers 1a and 1b is about 500 kHz. The control unit 58 outputs a transmission start signal to the drive circuit 54 and starts time measurement of the timer 56 at the same time. When receiving the transmission start signal, the drive circuit 54 drives the ultrasonic transmission receiver 1a and transmits an ultrasonic pulse. The transmitted ultrasonic pulse propagates through the flow rate measurement unit 51 and is received by the ultrasonic transmission receiver 1b. The received ultrasonic pulse is converted into an electrical signal by the ultrasonic transmission receiver 1 b and output to the reception detection circuit 55.
The reception detection circuit 55 determines the reception timing of the reception signal and stops the timer 56. The calculator 57 calculates the propagation time t1.
Next, the switching circuit 53 switches the ultrasonic transmission receivers 1 a and 1 b connected to the drive circuit 54 and the reception detection circuit 55. Then, again, the control unit 59 outputs a transmission start signal to the drive circuit 54 and simultaneously starts the time measurement of the timer 56. Contrary to the measurement of the propagation time t1, an ultrasonic pulse is transmitted by the ultrasonic transmission receiver 1b, received by the ultrasonic transmission receiver 1a, and the propagation time t2 is calculated by the calculation unit 57.
Here, the distance connecting the centers of the ultrasonic transmission receiver 1a and the supersonic transmission receiver 1b is L, the speed of sound in the absence of wind of LP gas is C, the flow velocity in the flow rate measurement unit 51 is V, The angle between the direction of the flow of the measurement fluid and the line connecting the centers of the ultrasonic transmission receivers 1a and 1b is θ.
The propagation times t1 and t2 are each obtained by measurement. Since the distance L is known, the flow rate V can be obtained by measuring the times t1 and t2, and the flow rate can be determined from the flow rate V.
In such an ultrasonic flowmeter, the propagation times t1 and t2 are measured by a method called a zero cross method. In this method, an appropriate threshold level is set for the received waveform as shown in FIG. 21 (a), and the time at which the amplitude becomes 0 after the threshold level is measured. When the S / N of the received signal is poor, the point at which the amplitude becomes 0 varies with time depending on the noise level, so t1 and t2 cannot be measured accurately, and it is difficult to measure the accurate flow rate. It may become.
When the ultrasonic transmitter / receiver of the present invention is used as an ultrasonic transducer of such an ultrasonic flowmeter, the S / N of the received signal is improved, and t1 and t2 can be measured with high accuracy. .
As shown in FIG. 21B, when the rising edge of the received signal is slower (narrow band) than in the case of FIG. 21A, t1 and t2 are measured with respect to the threshold level setting value. The peak position of the received signal may fluctuate, resulting in measurement errors. However, since the ultrasonic transmitter / receiver according to the present invention operates appropriately in a wide band, the reception signal rises well, and accurate flow rate measurement can be stably performed. In addition, it is preferable to use the average value of the value obtained by the measurement of other times as a value of t1 and t2.
The ability to transmit and receive broadband ultrasonic waves means that the signal falls early. For this reason, even when measurement is repeated quickly, it is not affected by the previous transmission / reception signal. As a result, even if the repetition frequency of measurement is increased, instantaneous measurement is possible, and gas leaks can be detected instantaneously.
In the above embodiments, the upper surface of the uppermost acoustic matching layer (first acoustic matching layer) is exposed, but this surface may be covered with a protective film having a thickness of 10 μm or less. Such a protective film avoids direct contact between the atmosphere and the acoustic matching layer, and contributes to maintaining the characteristics of the acoustic matching layer over a long period of time. The protective film is constituted by a film (not limited to a single layer) made of a material such as aluminum, silicon oxide, low-melting glass, or polymer. The protective film is deposited by sputtering or CVD.
Industrial applicability
According to the present invention, it is possible to put to practical use a high-performance ultrasonic transceiver that uses a thin acoustic matching layer formed of a material having extremely low acoustic impedance and low mechanical strength. Reliability in use is also improved. In the first aspect of the present invention, since the acoustic matching layer is protected and a protective portion that can also be used for defining the thickness of the acoustic matching layer is provided, the mechanical strength is low and the thin acoustic matching layer is provided. Can be formed with high accuracy and good reproducibility. As a result, a highly reliable ultrasonic transducer that can transmit and receive broadband ultrasonic waves with high sensitivity is provided. Further, in the second aspect of the present invention, a protective part (protective part / acoustic matching layer = protective matching layer) that also functions as an acoustic matching layer can be provided at an arbitrary position in the main surface of the piezoelectric body. By adjusting the sound speed and thickness of the two types of acoustic matching layers, it is possible to align the phases of the ultrasonic waves radiated from the two types of acoustic matching layers having different thicknesses and increase the transmission / reception sensitivity of the ultrasonic waves. .
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an ultrasonic transceiver according to Embodiment 1 of the present invention.
FIG. 2 is a top view of the ultrasonic transceiver according to the first embodiment of the present invention.
FIG. 3A is a graph showing the transmission / reception waveform of the ultrasonic transducer according to the first embodiment of the present invention, and FIG. 3B is a graph showing the transmission / reception waveform of the conventional ultrasonic transducer. .
FIG. 4 is a cross-sectional view schematically showing a case where the acoustic matching layer contracts in the first embodiment of the present invention.
FIG. 5 is a cross-sectional view of the ultrasonic transceiver according to the second embodiment of the present invention.
FIG. 6 is a cross-sectional view showing another configuration of the protection unit according to Embodiment 2 of the present invention.
FIG. 7 is a top view illustrating another configuration of the protection unit according to the second exemplary embodiment of the present invention.
FIG. 8 is a cross-sectional view of the ultrasonic transceiver according to the third embodiment of the present invention.
FIG. 9 is a cross-sectional view of an ultrasonic transceiver according to the fourth embodiment of the present invention.
FIG. 10 is a cross-sectional view of the ultrasonic transceiver according to the fifth embodiment of the present invention.
FIG. 11A is a graph showing the transmission / reception waveform of the ultrasonic transducer according to the fifth embodiment of the present invention, and FIG. 11B is a graph showing the transmission / reception waveform of the conventional ultrasonic transducer. .
FIG. 12 is a cross-sectional view showing another configuration of the lower acoustic matching layer according to Embodiment 6 of the present invention.
FIG. 13 is a cross-sectional view of the ultrasonic transceiver according to the seventh embodiment of the present invention.
FIG. 14 is a graph showing a transmission / reception waveform of the ultrasonic transducer according to the seventh embodiment of the present invention.
FIG. 15 is a cross-sectional view of an ultrasonic transceiver with another configuration according to Embodiment 7 of the present invention.
16 (a) to 16 (c) are process cross-sectional views illustrating a method of manufacturing the ultrasonic transducer shown in FIG.
17A is a cross-sectional view showing an acoustic matching layer when gel permeation is sufficient in the manufacturing process shown in FIG. 16, and FIG. 17B is an acoustic diagram when gel permeation is insufficient. It is sectional drawing which shows a matching layer.
18 (a) to 18 (d) are process cross-sectional views showing another method for manufacturing the ultrasonic transducer shown in FIG.
FIG. 19A and FIG. 19B are top views showing other examples of the configuration of the protection unit.
FIG. 20 is a sectional view of a tenth embodiment of an ultrasonic transducer according to the present invention.
FIG. 21 is a top view of the tenth embodiment of the ultrasonic transducer according to the present invention.
FIG. 22 is a schematic diagram showing ultrasonic interference in the tenth embodiment of the ultrasonic transducer according to the present invention.
FIG. 23 is a cross-sectional view schematically showing the phase of an ultrasonic wave propagated through the protective matching layer and the acoustic matching layer.
24 (a) to 24 (c) are process cross-sectional views showing a method for manufacturing the tenth embodiment of the ultrasonic transducer according to the present invention.
FIG. 25 is a transmission / reception waveform diagram of the tenth embodiment of the ultrasonic transducer according to the present invention.
FIG. 26 is a sectional view of an eleventh embodiment of the ultrasonic transducer according to the present invention.
FIG. 27 is a transmission / reception waveform diagram of the eleventh embodiment of the ultrasonic transducer according to the present invention.
FIG. 28 is a sectional view of a twelfth embodiment of an ultrasonic transducer according to the present invention.
29 (a) to 29 (d) are process cross-sectional views illustrating a method for manufacturing the twelfth embodiment of the ultrasonic transducer according to the present invention.
FIG. 30 is a sectional view of a thirteenth embodiment of an ultrasonic transducer according to the present invention.
FIGS. 31A and 31B are top views of the fourteenth embodiment of the ultrasonic transducer according to the present invention.
FIG. 32 is a sectional view of a fifteenth embodiment of an ultrasonic transducer according to the present invention.
FIG. 33 is a block diagram showing an ultrasonic flowmeter according to the sixteenth embodiment of the present invention.
FIG. 34A and FIG. 21B are graphs showing waveforms measured by the ultrasonic flowmeter of the present invention.
FIG. 35 is a sectional view showing a conventional ultrasonic flowmeter.
FIG. 36 is a cross-sectional view of a conventional ultrasonic transceiver.

【０２２９】
ここで、λは超音波が伝搬する気体中の波長であり、Ｌは超音波送受信器の超音波放射面からの距離である。例えば、周波数が５００ｋＨｚ、超音波伝搬媒体が空気（音速３４０ｍ／ｓ）、測定距離Ｌが１０ｍｍの場合、（式６）から、中心からの半径ｒが２．６〜３．７ｍｍ、４．６〜５．４ｍｍ、６．１〜６．７ｍｍ・・・の位置に保護部２を設けることが好ましいことがわかる。このような位置に保護部２を設けると、音の干渉による音場の乱れを防止し、近距離における超音波の感度劣化を防止するのに有効である。
【０２３０】
超音波送受信器の音波放射面の各点を、それぞれ、点音源に見立てると、それぞれの点音源から放射された球面波を合成したものが送信される超音波となる。超音波放射面からの距離が短い場合、位相の異なる超音波が相殺するため、高出力の超音波を送信することができない位置が存在する。超音波放射面から位相の同じ超音波のみを放射させるためには、位相の異なる超音波を放射する領域に保護部２を設けることが有効である。このような領域に保護部２が設けられると、位相の異なる超音波の放射を抑えることができるため、近距離における音場の乱れを防止して、高出力の超音波送信が可能となる。
【０２３１】
（実施形態１０）
図２０は、本発明による超音波送受波器の第１０の実施形態を示す断面図である。本実施形態の超音波送受波器２１は、圧電体２２と、圧電体２２の両面に設けられた電極２３ａ、２３ｂと、圧電体２２上に電極２３ａを介して設けられた保護整合層（第１音響整合部）２４と、圧電体２２上に電極２３ａを介して設けられた音響整合層（第２音響整合部）２５とを備えている。
【０２３２】
図２１は、図２０に示した超音波送受波器２１の上面図である。図２１からわかるように、本実施形態の超音波送受波器は、厚さ（高さ）の異なる保護整合層２４と音響整合層２５とが交互に同心円状に配置された構造を有している。
【０２３３】
本実施形態における圧電体２２は、圧電性を有する材料から形成され、厚さ方向に分極されている。圧電体２２の上下面に設けられた電極２３ａ、２３ｂに電圧が印加されると、電圧信号に基づいて圧電体２２で超音波が発生し、保護整合層２４および音響整合層２５を介して超音波伝搬媒体（気体など）２６へ放射される。また、超音波伝搬媒体２６を伝播してきた超音波は、保護整合層２４および音響整合層２５を介して圧電体２２へ伝播する。入射してきた超音波によって圧電体２２は変形し、電極２３ａと電極２３ｂとの間に電圧信号が発生する。
【０２３４】
圧電体２２の材料は任意であり、種々の公知材料から形成したものを用いることができる。圧電体２２の代わりに公知の電歪体を用いてもよい。電極２３ａ、２３ｂは好ましくは金属から形成されるが、金属以外の導電材料から形成されていても良い。
【０２３５】
保護整合層２４および音響整合層２５は、圧電体２２で発生した超音波振動を伝搬媒体２６へ効率よく伝搬させ、また、超音波伝搬媒体２６を伝搬してきた超音波を効率よく圧電体２２へ伝える機能を有している。
【０２３６】
本実施形態の音響整合層２５は、好ましくは、乾燥ゲルから形成される。乾燥ゲルは、ゾルゲル反応によって形成される多孔質体であり、密度ρと音速Ｃとの積（ρ×Ｃ）で規定される音響インピーダンスを極めて小さくすることが可能な材料である。このため、乾燥ゲルから形成した音響整合層２５を用いることにより、空気などの気体に対する超音波の送受波効率を極めて高くすることができる。
【０２３７】
乾燥ゲルは、湿潤ゲルを形成した後、この湿潤ゲルを乾燥することによって得られる。湿潤ゲルは、まず、ゲル原料液を用意し、このゲル原料液の反応によって湿潤ゲルを作製することができる。湿潤ゲルは、ゲル原料液の反応によって固体化した固体骨格部を有しており、この固体骨格部が溶媒を含んだ状態にある。
【０２３８】
湿潤ゲルを乾燥することによって得られる乾燥ゲルは、多孔質体であり、数ｎｍ〜数μｍ程度の固体骨格部の隙間に連続した気孔を有している。気孔の平均サイズは１ｎｍ〜数μｍ程度と極めて小さい。
【０２３９】
作製条件を調節して乾燥ゲルの密度を小さくしてゆくと、乾燥ゲルの固体部分における音速が極端に小さくなるとともに、細孔内の気体部分における音速も極端に小さくなる。そのため、乾燥ゲルの音速は、低密度状態で５００ｍ／秒以下の低い値を示し、極めて低い音響インピーダンスを示すことになる。特に固体骨格部および細孔径が数ｎｍ程度と小さいサイズを持つ乾燥ゲルは極めて低い音速を示す。また、ナノメートルサイズの細孔部では気体の圧損が大きいため、乾燥ゲルから音響整合層を形成した場合、音波を高い音圧で放射できる。
【０２４０】
後述する製造方法によれば、同じ原料を用いても製造プロセス条件を調節することにより、乾燥ゲルの音響インピーダンスを広い範囲内で任意に値に制御することができる。また、製造プロセス条件を変えることにより、密度が略同程度の大きさでありながら、音速だけを変化させた音響整合層を作製することも可能である。
【０２４１】
乾燥ゲルは、このような有利な特徴を有するが、機械的強度が低い。このため、製造歩留まりを高くすることが困難であり、使用時における信頼性も低かった。このように機械的強度の低い乾燥ゲルを保護する部材を設けることにより、製造歩留まりおよび信頼性が向上することは、実施形態１〜９について示したとおりである。
【０２４２】
実施形態１〜９における保護部は、超音波送受波器の製造歩留まり、あるいは使用時における信頼性を向上させるのに極めて有効であり、さらに音響整合層の厚さを高精度に制御しうるため、超音波送受波器の性能安定化に対して有効である。しかし、前述のように、圧電体が超音波を放射または受け取る面（主面）上に保護部を設けると、その保護部が音響的な障害となり得る。これは、また、実施形態１〜９では、音響整合層の材料とは異なる材料から形成される上記保護部の厚さが、音響整合層の厚さと略等しくなるように設定されているため、保護部と音響整合層との間で音速が異なり、音響整合層と略同じ厚さの保護部は音響整合層の役割をしないためである。このため、実施形態１〜９の保護部は、超音波の送受信に対して阻害する要因となり得るため、圧電体の主面の外側に配置することが好ましい。
【０２４３】
しかし、更に厳しい環境条件に対する信頼性の確保や、超音波送受波器の外径の制限などによっては、圧電体上部に保護部を設けざるを得ない場合がある。
【０２４４】
本実施形態では、圧電体の主面に、音響整合層２５を保護する機能を果たす保護部（密度が相対的に高く、音響整合層２５よりも機械的強度が高い材料から形成される）を有しながらも、超音波送受波器としての性能を損なわない構成を採用する。
【０２４５】
本実施形態では、圧電体２２の主面に設けられた保護部の厚さを送受信する超音波の波長の約１／４に設定している。これにより、機械的強度が相対的に高い保護部も音響整合層として機能する。このため、本明細書では、このような保護部を「保護整合層」と称する場合がある。このような構成を採用することにより、音響整合層を保護する保護部も音響整合層としての役割を果たすため、高感度な超音波送受波器を実現することができる。
【０２４６】
音響整合層としての機能を最もよく発揮する厚さは、超音波の波長の１／４である。一方、保護整合層２４における音速と音響整合層２５における音速は異なる。このため、保護整合層２４の厚さＬ３と音響整合層２５の厚さＬ１とは、図２０に示されるように、異なる大きさを有している（Ｌ３＞Ｌ１）。
【０２４７】
保護整合層２４および音響整合層２５の厚さがいずれも音速の１／４程度に設定されると、保護整合層２４の厚さが音響整合層２５の厚さと異なるため、音響整合層２５の上面から放射された超音波と、保護整合層２４の上面から放射された超音波が干渉する場合がある。高感度な超音波送受波器を実現するためには、それぞれから放射される超音波の位相関係が極めて重要となる。
【０２４８】
図２２（ａ）は、保護整合層２４の上面における超音波の波形を示し、図２２（ｂ）は、音響整合層２５の上方において、保護整合層２４の上面と同じレベルにおける超音波の波形を示している。なお、図２２（ｂ）における符号「ｔａ」は、超音波が超音波伝搬媒体２６を伝搬する時間を示している。各グラフにおける横軸の１目盛りは、超音波の周波数が５００ｋＨｚのとき、約３μ秒である。
【０２４９】
音響整合層２５の上面から放射された超音波は、気体などの超音波伝播媒体２６を通って保護整合層２４の上面と同じレベルに達する。このため、伝搬媒体２６における音速や伝播媒体２６のサイズＬ２によっては、音響整合層２５の上方において、保護整合層２４の上面と同じレベルにおける超音波の波形の位相関係が変化する。
【０２５０】
なお、図２２（ａ）および（ｂ）の信号波形は、保護整合層２４および音響整合層２５から放射される超音波の波長および振幅が等しいと仮定して求めたものである。
【０２５１】
保護整合層２４の厚さＬ３および音響整合層２５の厚さＬ１が、それぞれ、各層における超音波波長の１／４であるとき、保護整合層２４の下面と上面との間を超音波が伝搬するに要する時間は、音響整合層２５の下面と上面との間を超音波が伝搬するに要する時間に等しい。従って、音響整合層２５の上面から放射された超音波が保護整合層２４の上面と同じレベルの位置に達した超音波の位相は、保護整合層２４を伝搬して保護整合層２４の上面に達した超音波の位相に比べて、遅れている。この位相の遅れは、音響整合層２５の上面から出た超音波が伝搬媒体２６の中を距離Ｌ２だけ伝播する時間に対応している。
【０２５２】
送受信する超音波の周波数をｆ［秒^-1］とすると、超音波の１波長に等しい距離だけ超音波が進むのに必要な時間は１／ｆ［秒］である。超音波が本実施形態の保護整合層２４を通過するのに必要な時間ｔ３は、１／４ｆ［秒］である。一方、超音波が本実施形態の音響整合層２５を通過するのに必要な時間ｔ２も、１／４ｆ［秒］である。ここで、超音波が伝搬媒体２６の中をＬ２の距離だけ伝搬するために必要な時間をｔ２（＝ｔａ）とすると、時間ｔ２に依存して、保護整合層２４の上面から放射された超音波と音響整合層２５の上面から放射された超音波との間に干渉が発生する。この干渉により、超音波の波形および感度が変化する。
【０２５３】
図２２（ｃ）は、時間ｔ２が１／２ｆ［秒］である場合に観測される超音波波形を示しており、図２２（ｄ）は、時間ｔ２が１／ｆ［秒］である場合に観測される超音波波形を示している。図２２（ｃ）および（ｄ）からわかるように、時間ｔ２の値により、観測される超音波の感度が大きく異なる。時間ｔ２が１／２ｆ［秒］と等しいとき、位相のずれが超音波の半波長となり、観測される超音波の感度は低くなる。一方、時間ｔ２が１／ｆ［秒］のとき、位相のずれが超音波振動子の波長の整数倍となるため、観測される超音波の感度は高くなる。時間ｔ２が１／２ｆ〜１／ｆ［秒］の範囲内にあるとき、ｔ２が１／２ｆ［秒］から１／ｆ［秒］に近づくほど、超音波の送受信感度が上昇する。
【０２５４】
音響整合層２５から放射された超音波が伝搬媒体２６を伝搬して保護整合層２４の上面と同じレベルに達したとき、その超音波の位相が保護整合層２４を伝搬してきた超音波の位相と略一致するように音響整合層２５および保護整合層２４の厚さを調節すると、高感度の超音波送受波器を提供できる。なお、本明細書で「位相が略一致する」とき、超音波の位相の差が超音波波長の１／４以下になることを意味し、位相の差は小さいほど好ましい。
【０２５５】
図２３は、時間ｔ２が１／ｆ［秒］である場合の超音波の位相を模式的に示した断面図である。この図では、保護整合層２４の上面における超音波の位相と、音響整合層２５の上方であって保護整合層の上面と同レベルにおける超音波の位相とが一致している。このような位相の一致が生じしたとき、超音波の送受信感度が最大化される。なお、このような位相の完全な一致が生じない場合でも、位相のずれが少なく設定されると、超音波の送受信感度は従来よりも充分に向上される。位相のずれは、超音波伝搬媒体における超音波波長の１／４以下に調節されていることが好ましく、１／８以下に調節されていることが更に好ましい。
【０２５６】
音響整合層２５の厚さＬ１および保護整合層２４の厚さＬ３を、それぞれ、音響整合層２５および保護整合層２４における超音波波長の１／４程度に制御するだけでは、Ｌ２の大きさは（Ｌ３−Ｌ１）として一義的に決まるため、ｔ２を任意に設定することができない。このため、時間ｔ２が所望の大きさになるようにするには、音響整合層２５や保護整合層２４の厚さだけではなく、音響整合層２５や保護整合層２４における音速を適切に制御する必要がある。本発明の好ましい実施形態では、音響整合層２５を音速の制御が容易な乾燥ゲルから形成する。
【０２５７】
次に、図２４（ａ）から（ｃ）を参照しながら、本実施形態の超音波送受波器２１の製造方法の実施形態を説明する。本実施形態においては、超音波伝搬媒体２６として空気（密度：１．１８ｋｇ／ｍ³、音速：約３４０ｍ／ｓ、音響インピーダンス約４．０×１０²ｋｇ／ｍ²／ｓ）を考える。
【０２５８】
まず、図２４（ａ）に示すように、送受信する超音波の波長に合わせた圧電体２２を用意する。この段階の圧電体２２には、図２４（ａ）に示す保護整合層２４は設けられていない。圧電体２２としては、圧電セラミックスや圧電単結晶など圧電性の高い材料が好ましい。圧電セラミックスとしては、チタン酸ジルコン酸鉛、チタン酸バリウム、チタン酸鉛、ニオブ酸鉛などを用いることができる。圧電単結晶としては、チタン酸ジルコン酸鉛単結晶、ニオブ酸リチウム、水晶などを用いることができる。
【０２５９】
本実施形態では、圧電体２２としてチタン酸ジルコン酸鉛セラミックスを用い、送受信する超音波の波長を５００ｋＨｚに設定している。このような超音波を圧電体２２が効率よく送受信できるようにするため、圧電体２２の共振周波数を５００ｋＨｚに設計する。このため本実施形態では、直径が１２ｍｍ、厚さが約３．８ｍｍの円柱形状を有する圧電セラミックスから形成された圧電体２２を用いている。圧電体２２の両面には銀の焼付けによる電極２３ａ、２３ｂが形成され、この方向に分極処理が施されている。
【０２６０】
次に、保護整合層２４としての機能する３つのリング状部材を用意し、図２４（ａ）に示すように圧電体２２の主面に接合する。このとき、図２１に示すように、リング状部材の各中心が圧電体２２の中心に揃うようにする。保護整合層２４としての機能する３つのリング状部材は、それぞれ、外径１２ｍｍ、内径１１ｍｍ、厚さ１．０ｍｍの第１リング状部材、外径８ｍｍ、内径７ｍｍ、厚さ１．０ｍｍの第２リング状部材、および、外径４ｍｍ、内径３ｍｍ、厚さ１．０ｍｍの第３リング状部材である。
【０２６１】
本実施形態における保護整合層２４には、機械的強度が高く、音響整合層を保護できる機能が求められるだけでなく、音響整合層の機能を果たすために比較的低い音響インピーダンスを有することが求められる。このような材料として、本実施形態では、多孔質体のセラミックスを用いる。この多孔質セラミックスは、見かけ密度が０．６４×１０³ｋｇ／ｍ³、音速が２０００ｍ／ｓ、音響インピーダンスが約１．２８×１０⁶ｋｇ／ｍ³である。セラミックスとしては、チタン酸バリウム系の材料を用いている。なお、「見かけ密度」とは、多孔質体に含まれる空間部分をも含んだ密度である。多孔質セラミックスは、体積の約８０％程度が空間部分であり、セラミックスの実体部分は約２０体積％である。
【０２６２】
上述のように、保護整合層２４の音速が約２０００ｍ／ｓであるため、５００ｋＨｚにおける波長の１／４の厚さは１．０ｍｍに相当する。このため、本実施形態では保護整合層２４として機能するリング状部材の厚さを１．０ｍｍに設定している。
【０２６３】
本実施形態で用いる多孔質セラミックスは、次のようにして作製され得る。
【０２６４】
まず、樹脂製の微小なボールとセラミックス粉末を混合、加圧成形する。その後、セラミックスを焼結する。この焼結過程において、樹脂ボールは加熱され、燃焼して除去される。焼結に際して、加熱を急激に行うと、樹脂ボールが膨張または急激なガス化を起こし、セラミックス構造体を破壊してしまうおそれがある。このため、焼結は緩やかな加熱によって行うことが好ましい。
【０２６５】
本実施形態では、このような多孔質セラミックスから形成したの保護整合層２４と圧電体２２とを接着剤による接着によって接合する。例えば、接着剤としてはエポキシ系樹脂を用い、０．１ＭＰａ程度の圧力をかけながら、１５０℃の恒温槽中で２時間程度放置すると、接着剤は硬化し、保護整合層２４と圧電体２２とが接合する。
【０２６６】
次に、こうして形成した圧電体２２／保護整合層２４からなる複合体上に、図２４（ｂ）に示すように音響整合層２５を設ける。本実施形態では、音響整合層２５を乾燥ゲルから形成する。
【０２６７】
本実施形態では、まず、図２４（ｂ）に示す厚さの音響整合層２５を形成した後、図２４（ｃ）に示すようように音響整合層２５を薄くする。このとき、図２０に示す距離Ｌ２（＝Ｌ３−Ｌ１）が空気中に超音波の１波長に等しくなるように、保護整合層２４の厚さＬ３および音響整合層の厚さＬ１を設定する。具体的には、送受信する超音波の周波数が５００ｋＨであるので、この超音波の空気中における１波長は、０．６２ｍｍである。一方、保護整合層２４の厚さＬ３は１．０ｍｍであるため、音響整合層２５の厚さＬ１は、０．３２ｍｍ（＝１．０ｍｍ−０．６２ｍｍ）とになる。また、音響整合層２５が音響整合層として適切に機能するためには、この厚さＬ１（＝０．３２ｍｍ）が音響整合層２５を伝搬する超音波の波長の１／４となることが最も望ましい。従って、０．３２ｍｍが送受信する超音波の波長の１／４となるような音速を有する材料特性を有することが必要となる。計算によれば、音速が６４０ｍ／ｓとなるような乾燥ゲルから厚さ０．３２ｍｍの音響整合層２５を形成すれば良い。
【０２６８】
なお、保護整合層２４の厚さは、保護整合層２４における超音波波長の１／４であることが好ましいが、その大きさに限定されるわけではない。超音波波長の１／８以上１／３以下の範囲であれば良く、超音波波長の１／６以上１／４以下の範囲であることが更に好ましい。超音波の波長に分布がある場合、ピーク波長を基準にして厚さを決定することが好ましい。本明細書においては、波長に分布がある場合、「波長の１／４」とは「ピーク波長の１／４」を意味するものとする。
【０２６９】
音響整合層２５が単層である場合、音響整合層２５の厚さも、音響整合層２５における超音波波長の１／４であることが好ましいが、その大きさに限定されるわけではない。超音波波長の１／８以上１／３以下の範囲であれば良く、超音波波長の１／６以上１／４以下の範囲であることが更に好ましい。音響整合層が多層構造を有している場合、各構成層が上記の厚さを有していることが好ましい。音響整合層が多層構造を有している超音波送受波器は、実施形態２として後述する。
【０２７０】
音響整合層２５を構成する乾燥ゲルの材質としては、無機材料、有機高分子材料など様々な材料を用いることができる。無機材料の固体骨格部としては、酸化ケイ素（シリカ）、酸化アルミニウム（アルミナ）、酸化チタンなどを用いることができる。また有機材料の固体骨格部としては、一般的な熱硬化性樹脂、熱可塑性樹脂を用いることができ、例えば、ポリウレタン、ポリウレア、フェノール樹脂、ポリアクリルアミド、ポリメタクリル酸メチルなどを用いることができる。
【０２７１】
本実施形態では、音響整合層２５の材料として、コスト、環境安定性、製造のしやすさ、超音波送受波器の安定な温度特性などの観点から、固体骨格部として酸化ケイ素（シリカ）を持つ乾燥ゲルを採用する。
【０２７２】
６４０ｍ／ｓの音速は、乾燥ゲルの音速としては比較的高い値である。このため、本実施形態では、音響整合層２５として乾燥ゲル層を形成する際に、ゲル化工程（以下、「第１ゲル化工程」と称する。）に引き続いて乾燥工程を行う従来の製造方法ではなく、第１ゲル化工程後に第２ゲル化工程を行う方法を採用する。
【０２７３】
第２ゲル化工程を行わずに第１ゲル化工程だけを行う場合は、相対的に高い音速を示す乾燥ゲルを得ることが困難である。なお、乾燥ゲルの密度は、音速と略比例して高くなるため、「高い音速」は「高い密度」を意味する。ゲルの音速を高くする目的で、ゲル原料液中におけるゲル原料の濃度を高くすると、ゲル化反応が均一に進行せず、ランダムな音速分布を持つ湿潤ゲルが形成される。この湿潤ゲルを乾燥することによって得られる乾燥ゲルも、ランダムな密度分布を持つこととなる。このため、ゲル原料液中におけるゲル原料の濃度を高くすると、音速を均一化することは極めて難しくなる。
【０２７４】
本実施形態では、ゲルの不均一化を避けるため、第１ゲル化工程で形成する乾燥ゲルの音速は２００ｍ／ｓ程度以下に調節し、第２ゲル化工程によって密度を更に上昇させ、均一に音速を上昇させる。第２ゲル化工程では、第１ゲル化工程で得られた湿潤ゲルを再びゲル原料液（第２ゲル化原料液）に浸漬する処理を行う。そして、第２ゲル化工程では、第２ゲル化原料液中の触媒となるアンモニアの濃度を低く調整する。このため、第１ゲル化工程で得られた湿潤ゲルの外ではゲル化が起こらない。しかし、第１ゲル化工程で得られた湿潤ゲルの内部では、第１ゲル化工程で形成された骨格に第２ゲル化原料液が付着していくように成長する。このため、ゲル原料液自体がゲル化しない条件においても、この反応は進行する。このようにしてゲルの音速、密度を変化させることが可能となる。
【０２７５】
具体的には、以下に示す工程を行うことにより、乾燥ゲルによる音響整合層２５を形成する。
【０２７６】
工程１： 第１ゲル化ゲル原料液の用意
テトラエトキシシラン／エタノール／水／塩化水素を、モル比で１／２／１／０．０００７８で混合して、６５度の恒温槽中で３時間、テトラエトキシシランの加水分解を進行させる。更に、水／ＮＨ₃を、２．５／０．００５７の割合（テトラエトキシシランに対するモル比）を加えて混合したゲル原料液を用意する。
【０２７７】
工程２： 第１ゲル化工程
上記のようにして調整したゲル原料液（第１ゲル化原料液）を、圧電体２２と保護整合層２４で形成された空間に滴下する。この際、一番外側の保護整合層２４の外周にテフロン製のシートを巻きつけ、ゲル原料液がこぼれないように枠を形成する。
【０２７８】
ゲル原料液を滴下したサンプルを恒温槽中で水平を保ちながら５０℃で約１日放置する。こうして、圧電体２２と保護整合層２４とによって形成された空間内に供給されたゲル原料液がゲル化し、湿潤ゲルを形成する。
【０２７９】
工程３： 第２ゲル化工程（音速、密度の調整）
第１ゲル化工程で得られた音響整合層に対して、そのまま乾燥工程を行った場合、密度は２．０×１０²ｋｇ／ｍ³程度、音速は２００ｍ／ｓ程度となる。本実施形態では、音速、密度を更に高くする目的で第２ゲル化工程を行う。
【０２８０】
まず、第１ゲル化工程で得られた湿潤ゲルをエタノールで洗浄し、第２ゲル化原料液を準備する。第２ゲル化原料液として、テトラエトキシシラン／エタノール／０．１規定アンモニア水を、体積比で６０／３５／５を混合したものを用いる。
【０２８１】
第１ゲル化工程で得られた圧電体２２／湿潤ゲル／保護整合層２４からなる複合体を密閉容器中の第２ゲル化原料液に浸漬し、７０℃の恒温槽中で約４８時間放置する。この第２ゲル化工程により第１ゲル化工程で得られたゲル骨格が成長し、密度、音速が高くなる。
【０２８２】
工程４： 疎水化工程
疎水化工程は、必ずしも必要ではないが、吸湿により性能が劣化することがあるため、行うことが好ましい。疎水化工程は、第２ゲル化工程の後、湿潤ゲル内に残留している、第２ゲル化原料液をエタノールにより置換・洗浄した後、ジメチルジメトキシシラン／エタノール／１０重量％アンモニア水を、重量比で４５／４５／１０の割合で混合して得られた疎水化液に、４０℃で、約１日間、浸漬することによって、疎水化工程を行う。
【０２８３】
工程５： 乾燥工程
以上の工程で得られた湿潤ゲルから、乾燥ゲルを得るために、乾燥工程を行う。本実施形態では、乾燥方法として、超臨界乾燥法を用いる。乾燥ゲルは前述のように、非常に小さなナノメートルサイズ程度の多孔質体であり、骨格部分の太さや、結合の強さ、空孔の大きさによっては、湿潤ゲルから乾燥ゲルへの溶媒乾燥の際に、溶媒の表面張力によって、破壊されてしまうことがある。
【０２８４】
このため、表面張力の働かない超臨界乾燥法が有用に利用することができる。具体的には、上述の疎水化液をエタノールで置換した後、以上の工程で得られた圧電体２２／湿潤ゲル／保護部研音響整合層２５の複合体を耐圧容器に入れて、湿潤ゲル内のエタノールを液化二酸化炭素に置換する。
【０２８５】
更に容器内にポンプで液化二酸化炭素を送り込むことにより、容器内の圧力を１０ＭＰａまで上昇させる。その後、５０℃まで昇温することで容器内を超臨界状態とした。次に温度を５０℃に保ったまま、圧力をゆっくり開放することで乾燥を完了する。
【０２８６】
工程６： 厚さ調整工程
こうして形成した乾燥ゲル層を、旋盤によりその厚さを０．３２ｍｍとなるように音響整合層２５のみの部分を研削した。
【０２８７】
このようにして得られた音響整合層２５を形成する乾燥ゲルの密度は、約０．６×１０³ｋｇ／ｍ³であり、音速は約６４０ｍ／ｓとなる。また保護整合層２４の一部に、音響整合層２５となる乾燥ゲルが浸透しているが、保護整合層２４の音速には影響を与えない。
【０２８８】
乾燥ゲルから音響整合層を形成する工程の前に、電極２３ｂと音響整合層２５との密着性が良くなるように電極２３ｂの表面を処理するすることが好ましい。表面処理によって電極２３ｂと音響整合層２５との密着性が増すと、信頼性が更に向上する。このような表面処理としては、乾燥ゲルと化学的な結合をしやすい圧電体表面の電極に水酸基が付与されるようなプラズマ処理などを採用することができる。あるいは、電極２３ｂの表面に物理的な凹凸を形成することによってアンカー効果を付与することも有効である。具体的には、化学的および／または物理的なエッチング処理を好適に採用することができる。
【０２８９】
本実施形態では、音響整合層２５となるゲルを形成した後、旋盤による研削を行い、乾燥ゲルの厚さを調整する。厚さの調節は、第１ゲル化工程の際に滴下する第１ゲル化原料液の量（高さ）を調整することによって行ってもよい。この場合には、形成される音響整合層の厚さが最終的に０．３２ｍｍ程度となるように、３３．９μＬのゲル原料液をマイクロピペットで正確に量り取り、圧電体２２上に滴下する。保護整合層２４が８０％の空隙を有する多孔質体であるため、多孔質体に吸収される体積を換算して滴下量を計算する必要がある。
【０２９０】
このようにして製造した超音波送受波器の送受信波形を図２５に示す。図２５において、本実施形態の超音波送受波形は実線で示され、保護部と音響整合層２５とが同じ厚さを有する超音波送受波器（比較例）の超音波送受信波形は点線で示されている。図２５からわかるように、本実施形態によれば、信号の振幅が増加する。
本発明の構造を用いることで高感度化を達成できる。
【０２９１】
本実施形態では、保護整合層２４の上面と同一レベルの位置における超音波の位相を揃えるため、音響整合層２５および伝搬媒体２６を伝搬してきた超音波が、保護整合層２４を伝搬してきた超音波に比べて、ちょうど波長分だけ位相の遅れを生じるようにしている。さらに大きな音速を有する材料から保護整合層２４を形成する場合や、保護整合層２４の厚さＬ３を大きく設定する場合には、伝搬媒体２６による位相遅れを超音波波長の２波長以上に設定してもよい。
【０２９２】
（実施形態１１）
次に、図２６を参照しながら、本発明の超音波送受波器の第１１の実施形態を説明する。本実施形態の主な特徴点は、音響整合層が下層の第１音響整合層２５ａおよび上層の第２音響整合層２５ｂを含む積層構造を有している点である。
【０２９３】
音響整合層２５が２層構造を有する場合にも、各音響整合層２５ａ、２５ｂの厚さのそれぞれを、各音響整合層における超音波波長の１／４程度に設定することが好ましい。
【０２９４】
本実施形態でも、第１０の実施形態と同様に、保護整合層２４の上面と同一レベルの位置において超音波の位相を揃えるため、音響整合層２５および伝搬媒体２６を伝搬してきた超音波が、保護整合層２４を伝搬してきた超音波に比べて、超音波波長の略整数倍分だけの位相の遅れを生じるようにしている。
【０２９５】
図２６に示す構成では、超音波が保護整合層２４を伝播して保護整合層２４の上面に達したとき、その超音波と同位相の超音波は第１音響整合層と第２音響整合層２５ｂの境界面に達している。これは、第１音響整合層２５ａの音速が保護整合層２４における音速よりも小さいためである。超音波が第１音響整合層２５ａの上面から第２音響整合層２５ｂの上面に達するまでに、更に１／４ｆ［秒］の時間がかかる。このため音響整合層２５ｂの上面から伝搬媒体２６を伝搬して保護整合層２４の上面と同じレベルに達するまでの時間が３／４ｆ［秒］となるように設定すると、保護整合層２４の上面レベルで位相が揃う。このような構成を採用すると、音響整合層２５ａおよび２５ｂを透過して放射された超音波と、保護整合層２４を透過して放射された超音波との間に、１波長分のずれが生じ、両超音波が干渉して強め合うため、超音波の振幅が大きくなる。
【０２９６】
本実施形態における音響整合層２５ａ、２５ｂの製造方法を説明する。
【０２９７】
まず、第１０の実施形態における音響整合層２５の製造方法と同様にして、保護整合層２４を作製する。保護整合層２４の材料として多孔質セラミックスを用い、その厚さ（Ｌ７）を１．０ｍｍに設定する。
【０２９８】
本実施形態では、空気などの伝搬媒体２６を伝搬する時間が３／４ｆ［秒］となるように、第２音響整合層２５ｂの上面から保護整合層２４の上面レベルまでの距離（Ｌ６）を０．５１ｍｍに設定する。この結果、第１音響整合層２５ａと第２音響整合層２５ｂの合計厚さ（Ｌ４＋Ｌ５）は、０．４９ｍｍに等しくなる。
【０２９９】
本実施形態では、第２音響整合層２５ｂの音速を２００ｍ／ｓに設定すると、第２音響整合層２５ｂの厚さ（Ｌ５）は０．１０ｍｍに設定することが好ましい。Ｌ５＝０．１０ｍｍとすると、第１音響整合層２５ａの厚さ（Ｌ４）は０．３９ｍｍ（＝０．４９ｍｍ−０．１０ｍｍ）となる。第１音響整合層２５ａの厚さが第１音響整合層２５ａにおける超音波の１／４波長に相当するようにするには、第１音響整合層２５ａの音速を７８０ｍ／ｓにする必要がある。
【０３００】
次に、上記のような２層の音響整合層２５ａ、２５ｂの作製方法を説明する。この方法で特徴的な点は、第１０の実施形態で行った第２ゲル化工程を２度行うことにある。すなわち、本実施形態では、第１ゲル化工程で形成された湿潤ゲルの外側ではゲル化をしない第２ゲル化工程（第２−１ゲル化工程）を行った後に、湿潤ゲルの外側でもゲル化が生じる第２ゲル化工程（第２−２ゲル化工程）を行う。
【０３０１】
本実施形態では、まず、第１０の実施形態で行った工程１〜工程６と同様の工程を行うことにより、第１音響整合層２５ａを形成する。ただし、このときの第２ゲル化工程は、第２−１ゲル化工程である。
【０３０２】
この後に行う第２−２ゲル化工程における音響インピーダンスの増加を見込み、第２−１ゲル化工程では、第１音響整合層２５ａの密度約０．５×１０³ｋｇ／ｍ³、音速５００ｍ／ｓ程度となるように処理時間を調節する。本実施形態では、処理時間を第１０の実施形態における第２ゲル化工程の処理時間よりも短縮し、約３６時間に設定する。
【０３０３】
次に、第２−２ゲル化工程を行うことにより、第１音響整合層２５ａの音響インピーダンスを増加ざせるとともに、第１音響整合層２５ａの上部に第２音響整合層２５ｂを形成する。この第２−２ゲル化工程は、具体的には、以下のようにして行った。
【０３０４】
第２−２ゲル化工程：
まず、第２−２ゲル化原料液として、テトラエトキシシラン／エタノール／０．０５規定アンモニア水をモル比で、１／４／３の割合で混合した液を用意する。この第２−２ゲル化原料液を、第１音響整合層２５ａと保護整合層２４によって形成された空間内に充填する。次に、このまま室温で約２４時間放置することにより、ゲル化を完了する。こうして、第１音響整合層２５ａの音響インピーダンスを調整するとともに、第２音響整合層２５ｂとなる湿潤ゲルを形成する。
【０３０５】
この後、第１０の実施形態と同様にして、疎水化工程、乾燥工程、および厚さ調整工程を行うことにより、音響整合層２５ａ、２５ｂを完成する。本実施形態における音響整合層２５ａ、２５ｂは、以下のように特徴付けられる。
【０３０６】
第１音響整合層２５ａ
密度：０．７×１０³ｋｇ／ｍ³、音速：７８０ｍ／ｓ
音響インピーダンス：５．４６×１０⁵ｋｇ／ｍ²／ｓ
厚さ：０．３９ｍｍ
第２音響整合層２５ｂ
密度：０．２×１０³ｋｇ／ｍ³、音速：２００ｍ／ｓ
音響インピーダンス：４．０×１０⁴ｋｇ／ｍ²／ｓ
厚さ：０．１０ｍｍ
【０３０７】
本実施形態の超音波送受信器の送受信波形を図２７に示す。図２７において、本実施形態の超音波送受波器の超音波送受波形は実線で示され、音響整合層と保護整合層の厚さを等しくした超音波送受波器（比較例）の送受波形形は点線で示される。図２７からわかるように、本実施形態の超音波送受波器によれば、高感度化を実現することができる。
【０３０８】
本実施形態では、音響整合層２５を２層とした構成としたが、３層以上としても、保護整合層２４の上面部分で、超音波の位相が揃うように設計することで同様の効果が得られる。
【０３０９】
（実施形態１２）
図２８を参照しながら、本発明による超音波送受波器の第１２の実施形態を説明する。本実施形態の特徴的な点は、第１音響整合層２５ａと保護整合層２４とが、同じ材料によって一体的に形成されている点にある。第１音響整合層２５ａの上部には乾燥ゲルから形成した第２音響整合層２５ｂが形成されている。
【０３１０】
本実施形態では、第１音響整合層２５ａおよび保護整合層２４における超音波の音速や波長が等しく、しかも、保護整合層２４の厚さＬ１１を超音波の１／４波長に設定する。このため、第１音響整合層２５ａの厚さＬ８は超音波の１／４波長よりも小さい。第１音響整合層２５ａの厚さＬ８は、第２音響整合層２５ｂの厚さＬ９と、第２音響整合層２５ｂの上面から保護整合層２４の上面レベルまでの距離Ｌ１０によって決まる。
【０３１１】
本実施形態においても、音響整合層２５ａおよび５ｂを透過して放射された超音波と、保護整合層２４を透過して放射された超音波との間に、超音波波長の整数倍の位相遅れが生じる構成を採用している。このため、保護整合層２４の上面レベルにおいて、音響整合層２５ａ、２５ｂおよび伝搬媒体２６を伝搬してきた超音波の位相が揃う。
【０３１２】
音響整合層２５ａ、２５ｂを透過してきた超音波の感度を高めるには、第１音響整合層２５ａの厚さよりも第２音響整合層２５ｂの厚さが重要である。本実施形態では、第２音響整合層２５ｂの厚さは、送受信する超音波の波長の約１／４に設定する。第１音響整合層２５ａの厚さも、感度に影響を与えるが、その影響の多くは周波数の比帯域に及ぶ。
【０３１３】
このため、本実施形態では、まず材質の機械的強度などの点から、第２音響整合層２５ｂを構成しうる乾燥ゲル層の特性を決定する。次に、保護整合層２４と同じ材料から形成する第１音響整合層２５ａの厚さＬ８と、音波伝搬媒体の厚さＬ１０とを設定する。
【０３１４】
本実施形態では、保護整合層２４の材料として、前述の実施形態と同様に多孔質セラミックスを用い、その厚さ（Ｌ１１）を超音波波長の１／４に設定する。すなわち、Ｌ１１を１．０ｍｍに設定する。この場合、上記の多孔質セラミックスから形成される第１音響整合層２５ａの音速は２００ｍ／ｓ、密度は０．２×１０³ｋｇ／ｍ³となる。乾燥ゲルから形成する第２音響整合層２５ｂの厚さを超音波波長の１／４に設定するため、Ｌ９を０．１０ｍｍとする。
【０３１５】
このとき、以下の式７が成立する。
【０３１６】
Ｌ８＋Ｌ１０＝０．９［ｍｍ］・・・（７）
【０３１７】
式７は、Ｌ１１を１．０ｍｍ、Ｌ９を０．１ｍｍに設定したことから導かれる。
【０３１８】
優れた特性を発揮するには、以下の式が成立することが好ましい。
【０３１９】
Ｌ８／１＋Ｌ１０／（１７／２５）＝１［波長］・・・（８）
【０３２０】
本実施形態では、周波数が５００ｋＨｚの超音波を送受信するため、音響整合層２５ａにおける超音波の１波長は１．０ｍｍ、音波伝搬媒体２６における超音波の１波長は１７／２５ｍｍとなる。式８は、超音波の１波長に対する第１音響整合層２５ａの厚さＬ８の比率と、超音波の１波長に対する伝搬媒体２６の厚さＬ１０の比率との和である。式８を満足するということは、超音波が第１音響整合層２５ａおよび音波伝搬媒体２６を透過する際に１波長だけ進むことを意味している。言い換えると、超音波が感じる第１音響整合層２５ａおよび音波伝搬媒体２６の実効的な厚さが１波長分であることを意味する。
【０３２１】
式７および式８を満足するＬ８およびＬ１０を算出すると、Ｌ８＝約０．６９ｍｍ、Ｌ１０＝０．２１ｍｍとなる。
【０３２２】
次に、図２９（ａ）から（ｄ）を参照しながら、本実施形態の超音波送受波器の製造方法を説明する。
【０３２３】
まず、図２９（ａ）に示すように、多孔質セラミックスからなる厚さ１．０ｍｍのペレットを用意し、このペレットを図２９（ｂ）に示すように加工する。本実施形態では、ペレットの上面に溝を形成し、溝底部の厚さを０．６９ｍｍに調節する。この溝底部が第１音響整合層２５ａとして機能する部分である。溝は図２１に示すようにリング状に形成する。
【０３２４】
次に、図２９（ｃ）に示すように、溝の内部に第２音響整合層２５ｂを形成する。音響整合層２５ｂの厚さが０．１ｍｍになるようにする。保護整合層２４／音響整合層２５ａ、２５ｂの複合体を、図２９（ｄ）に示すように、圧電体２２に接合し、超音波送受波器２１を形成する。
【０３２５】
第１音響整合層２５ｂは、テトラメトキシシラン／エタノール／０．０５規定アンモニア水をモル比で、１／７／４の割合で混合した液を用いて、実施形態１と同様にして第１ゲル化工程を行うことによって形成する。
【０３２６】
保護整合層２４／音響整合層２５ａ、２５ｂからなる複合体の圧電体への接着は、実施形態１と同様に、エポキシ系の接着剤によって行うことができる。
【０３２７】
本実施形態によれば、保護整合層２４と音響整合層２５ａを一括的に形成できるため、製造工程を簡単にし、製造コストを低減できる。
【０３２８】
（実施形態１３）
図３０を参照しながら、本発明による超音波送受波器の第１３の実施形態を説明する。本実施形態に特徴的な点は、構造支持体を有している点である。
【０３２９】
本実施形態の超音波送受波器は、圧電体２２と第１音響整合層２５や保護整合層２４との間に構造支持体２７を有している点を除けば、実施形態１の超音波送受波記録媒体の構成と同様の構成を有している。
【０３３０】
構造支持体２７は音響整合層２５などが固定される円盤状支持部と、この円盤状支持部から軸方向に連続的に伸びる円筒部とを備えている。円筒部の端面は、断面がＬ字型に折れ曲がり、圧電体２２の遮蔽のためのプレート（不図示）や、他の装置などに固定しやすくなっている。
【０３３１】
構造支持体２７の表面には音響整合層２５や保護整合層２４が配置されており、支持部裏面には圧電体２２が配置されている。このような構造支持体２７を用いることにより、超音波送受波器の取り扱いが極めて容易となる。
【０３３２】
構造支持体は、密閉可能な容器（センサケース）から構成することができる。この場合、構造支持体２７の円筒部の開放端を遮蔽プレートなどで塞ぎ、かつ、構造支持体２７の内部を不活性ガスで満たせば、流量測定の対象とする流体から圧電体２２を遮断することができる。
【０３３３】
圧電体２２には電圧が印加されるため、可燃性ガスなどと圧電体が接すると、可燃性ガスに引火する危険性もある。しかし構造支持体２７を密閉性の容器から構成し、圧電体２２のある内部を外部流体などと遮断することによって、そのような引火を防止して、可燃性ガスなどに対しても安全に超音波を送受波することができる。
【０３３４】
また可燃性ガスでなくとも、圧電体２２と反応し、圧電体２２に特性の劣化を与える可能性のあるガスとの間で超音波を送受波する場合でも、圧電体２２が外部ガスから遮断することが好ましい。そうすることにより、圧電体２２の劣化を防止し、長期間に渡って信頼性の高い動作を実現することが可能となる。
【０３３５】
構造支持体２７のうち、圧電体２２と音響整合層２５や保護整合層２４との間に位置する部分は音響整合層として機能しない。このため、構造支持体２７が音響的な阻害として働かないようにするため、構造支持体２７のうち、圧電体２２と音響整合層２５や保護整合層２４との間に位置する部分の厚さを、送受波する超音波の波長の１／８程度以下とすることが望ましい。
【０３３６】
本実施形態では、構造支持体２７をステンレスから形成し、上記部分の厚さを０．２ｍｍに設定している。
【０３３７】
ステンレスの音速は約５５００ｍ／秒であり、超音波の５００ｋＨｚにおける波長は約１１ｍｍとなる。０．２ｍｍの厚さは波長の約１／５５に相当するため、構造支持体２７の存在は殆ど音響的阻害要因にはならない。
【０３３８】
構造支持体２７の材料は、ステンレスなどの金属材料に限定される物ではなく、セラミック、ガラス、樹脂などから目的に応じた材料が選択される。本実施形態では、外部の流体と圧電体を確実に分離し、構造支持体に何らかの機械的な衝撃が加わったとしても、圧電体と外部流体との接触を防止できる強度を与えるため、金属材料から構造支持体２７を作製している。これにより、例えば可燃性や爆発性を有するガスを対象として超音波の送受波を行っても高い安全性を確保することができる。
【０３３９】
なお、安全な気体に対して超音波の送受波を行う場合には、コスト低減を目的として、樹脂などの材料からなる構造支持体を用いても良い。
【０３４０】
（実施形態１４）
図３１（ａ）および（ｂ）を参照しながら、本発明による超音波送受波器の第１４の実施形態を説明する。図３１（ａ）および（ｂ）は、本実施形態の上面図である。
【０３４１】
図２１に示す例では、保護整合層２４として機能する多孔質セラミック製のリング（同じ幅で直径の異なる３つのリング状部材）を用い、それらの中心が一致するように圧電体主面に配置しているが、図３１（ａ）に示すように幅の異なるリングを用いて保護整合層２４を形成しても良い。また、図３１（ｂ）に示すように、島状の保護整合層２４をランダムに配置してもよい。
【０３４２】
保護整合層２４と音響整合層２５とが、超音波送受波器の主面上に規則的に配列されている場合、その主面に対して或る角度を持った方向に超音波の位相が揃い、振幅が強まる。これは、「サイドローブ」と呼ばれ、超音波計測を行う上で阻害要因となる。しかし、図３１に示すように、保護整合層２４の配列が周期性を持たない構成を採用することにより、サイドローブを抑制し、精度および信頼性の高い超音波計測を可能とすることができる。
【０３４３】
（実施形態１５）
図３２を参照しながら、本発明による超音波送受波器の第１５の実施形態を説明する。
【０３４４】
本実施形態の超音波送受波器は、保護整合層２４の厚さが面内分布を有している点に第１の特徴点を有している。前述の各実施形態では、保護整合層２４の厚さが面内で一様に設定されているが、本実施形態では、意図的に面内分布が与えられている。また、本実施形態の第２の特徴点は、圧電体２２上に設けられた保護整合層２４が異なる２種類の材料から形成されていることにある。
【０３４５】
本実施形態の構成によれば、異なる材料の採用および／または異なる厚さの面内分布を付与することにより、サイドローブを抑制したり、送受信する超音波の周波数を変化させて広帯域化することができる。
【０３４６】
なお、各保護整合層２４の厚さは、超音波波長の１／８以上１／３以下の範囲内に含まれていることが好ましく、超音波波長の１／６以上１／４以下の範囲内に含まれていることが更に好ましい。ただし、異なる厚さを有する保護整合層２４の一部の厚さが上記の範囲から外れていてもよい。上記の範囲から外れた厚さを有する保護整合層２４は、音響整合層としては機能しないため、超音波送受信の感度が低下する。しかし、音響整合層として機能しない保護層（もはや「保護整合層」とは呼べない）を圧電体上の適当な位置に置くことにより、近距離における超音波場の乱れを防止して良好な超音波計測を可能にすることができる。
【０３４７】
（実施形態１６）
図３３を参照しながら、本発明による超音波流量計の実施形態を説明する。
【０３４８】
本実施形態の超音波流量計は、流量測定部５１として機能する管内を被測定流体が速度Ｖで流れるよう設置される。流量測定部５１の管壁５２には、本発明の超音波送受信器から形成した超音波送信受波器１ａおよび１ｂが相対して配置されている。
【０３４９】
ある時点では、超音波送受信器１ａが超音波送波器として機能し、超音波送受信器１ｂを超音波受波器として機能するが、他の時点では、超音波送受信器１ａが超音波受波器として機能し、超音波送受信器１ｂを超音波送波器として機能する。この切り替えは、切替回路５３によって行われる。
【０３５０】
超音波送受信器１ａ、１ｂは、切替回路５３を介して、超音波送受信器１ａおよび１ｂを駆動する駆動回路５４と、超音波パルスを検知する受信検知回路５５とに接続されている。受信検知回路５５の出力は、超音波パルスの伝搬時間を計測するタイマ５６に送られる。タイマ５６の出力は、流量を演算する演算部５７に送られる。演算部５７では、測定された超音波パルスの伝搬時間に基づいて、流量測定部５１内を流れる流体の速度Ｖが計算され、流量が求められる。駆動回路５４およびタイマ５６は、制御部５８に接続され、制御部５８から出力された制御信号によって制御される。
【０３５１】
以下、この超音波流量計の動作をより詳細に説明する。
【０３５２】
被測定流体として、ＬＰガスが流量測定部５１を流れる場合が考える。超音波送信受波器１ａおよび１ｂの駆動周波数を約５００ｋＨｚとする。制御部５８は、駆動回路５４に送信開始信号を出力すると同時に、タイマ５６の時間計測を開始させる。駆動回路５４は、送信開始信号を受けると、超音波送信受波器１ａを駆動し、超音波パルスを送信する。送信された超音波パルスは流量測定部５１内を伝搬して、超音波送信受波器１ｂで受信される。受信された超音波パルスは超音波送信受波器１ｂで電気信号に変換され、受信検知回路５５に出力される。
【０３５３】
受信検知回路５５は、受信信号の受信タイミングを決定し、タイマ５６を停止させる。演算部５７は、伝搬時間ｔ１を演算する。
【０３５４】
次に、切替回路５３により、駆動回路５４および受信検知回路５５に接続する超音波送信受波器１ａおよび１ｂを切り替える。そして、再び、制御部５９は駆動回路５４に送信開始信号を出力すると同時に、タイマ５６の時間計測を開始させる。伝搬時間ｔ１の測定と逆に、超音波送信受波器１ｂで超音波パルスを送信し、超音渡送信受波器１ａで受信し、演算部５７で伝搬時間ｔ２を演算する。
【０３５５】
ここで、超音波送信受波器１ａと超音渡送信受波器１ｂの中心を結ぶ距離をＬ、ＬＰガスの無風状態での音速をＣ、流量測定部５１内での流速をＶ、被測定流体の流れの方向と超音波送信受波器１ａおよび１ｂの中心を結ぶ線との角度をθとする。
【０３５６】
伝搬時間ｔ１、ｔ２は、それぞれ、測定によって求められる。距離Ｌは既知であるので、時間ｔ１とｔ２を測定すれば、流速Ｖが求められ、その流速Ｖから流量を決定することができる。
【０３５７】
このような超音波流量計において、伝搬時間ｔ１、ｔ２はゼロクロス法と呼ばれる方法によって測定される。この方法では、図３４（ａ）に示すような受信波形に対して、適切なスレッショルドレベルを設定し、そのスレッショルドレベルを超えて次に振幅が０となる点の時間を測定する。受信信号のＳ／Ｎが悪い場合、ノイズレベルによっては振幅が０となる点が時間的に変動するため、正確にｔ１、ｔ２を測定することができず、正確な流量を測定することが困難になる場合がある。
【０３５８】
このような超音波流量計の超音波送受波器として、本発明の超音波送受信器を用いると、受信信号のＳ／Ｎが向上し、ｔ１、ｔ２を高い精度で測定することが可能となる。
【０３５９】
図３４（ｂ）に示すように、図３４（ａ）の場合に比べて、受信信号の立ち上がりが遅い（狭帯域である）と、スレッショルドレベルの設定値に対して、ｔ１、ｔ２を測定する受信信号の山の位置が変動し、測定誤差となる可能性がある。しかし、本発明による超音波送受信器は広帯域で適切に動作するため、受信信号の立ち上がりがよく、正確な流量測定を安定的に行うことが可能となる。なお、ｔ１、ｔ２の値としては、他数回の測定によって得られた値の平均値を用いることが好ましい。
【０３６０】
広帯域の超音波を送受信できるということは、信号の立下りも早いことを意味する。このため、測定の繰り返しを速く行った場合でも、前の送受信信号の影響を受けることが無い。その結果、測定の繰り返し周波数を高くしても、瞬時の計測を可能とするものであり、ガスもれなどを瞬時に検知することが可能となる。
【０３６１】
以上の各実施形態では、最上層の音響整合層（第１音響整合層）の上面は露出しているが、この面を厚さ１０μｍ以下の保護膜でカバーしても良い。このような保護膜は、大気と音響整合層との直接的な接触を避け、音響整合層の特性を長期にわたって保持するのに寄与する。保護膜は、例えば、アルミニウム、酸化ケイ素、低融点ガラス、高分子などの材料からなる膜（単層に限定されない）によって構成される。保護膜は、スパッタリング法やＣＶＤ法などによって堆積される。
【０３６２】
【図面の簡単な説明】
【０３６３】
【図１】 本発明の実施形態１における超音波送受信器を示す断面図である。
【図２】 本発明の実施形態１における超音波送受信器の上面図である。
【図３】 （ａ）は、本発明の実施形態１における超音波送受波器の送受信波形を示すグラフであり、（ｂ）は、従来の超音波送受波器の送受信波形を示すグラフである。
【図４】 本発明の実施形態１において、音響整合層が収縮した場合を模式的に示す断面図である。
【図５】 本発明の実施形態２における超音波送受信器の断面図である。
【図６】 本発明の実施形態２における保護部の他の構成を示す断面図である。
【図７】 本発明の実施形態２における保護部の他の構成を示す上面図である。
【図８】 本発明の実施形態３における超音波送受信器の断面図である。
【図９】 本発明の実施形態４における超音波送受信器の断面図である。
【図１０】 本発明の実施形態５における超音波送受信器の断面図である。
【図１１】 （ａ）は、本発明の実施形態５における超音波送受波器の送受信波形を示すグラフであり、（ｂ）は、従来の超音波送受波器の送受信波形を示すグラフである。
【図１２】 本発明の実施形態６における下層音響整合層の他の構成を示す断面図である。
【図１３】 本発明の実施形態７における超音波送受信器の断面図である。
【図１４】 本発明の実施形態７における超音波送受波器の送受信波形を示すグラフである。
【図１５】 本発明の実施形態７における別構成の超音波送受信器の断面図である。
【図１６】 （ａ）から（ｃ）は、図１２に示す超音波送受波器の製造方法を示す工程断面図である。
【図１７】 （ａ）は、図１６に示す製造工程において、ゲルの浸透が充分な場合の音響整合層を示す断面図であり、（ｂ）は、ゲルの浸透が不充分な場合の音響整合層を示す断面図である。
【図１８】 （ａ）から（ｄ）は、図１２に示す超音波送受波器の他の製造方法を示す工程断面図である。
【図１９】 （ａ）および（ｂ）は、それぞれ、保護部の他の構成例を示す上面図である。
【図２０】 本発明による超音波送受波器の第１０の実施形態の断面図である。
【図２１】 本発明による超音波送受波器の第１０の実施形態の上面図である。
【図２２】 本発明による超音波送受波器の第１０の実施形態における超音波の干渉を示す模式図である。
【図２３】 保護整合層および音響整合層を伝搬した超音波の位相を模式的に示す断面図である。
【図２４】 （ａ）から（ｃ）は、本発明による超音波送受波器の第１０の実施形態を製造する方法を示す工程断面図である。
【図２５】 本発明による超音波送受波器の第１０の実施形態の送受信波形図である。
【図２６】 本発明による超音波送受波器の第１１の実施形態の断面図である。
【図２７】 本発明による超音波送受波器の第１１の実施形態の送受信波形図である。
【図２８】 本発明による超音波送受波器の第１２の実施形態の断面図である。
【図２９】 （ａ）から（ｄ）は、本発明による超音波送受波器の第１２の実施形態を製造する方法を示す工程断面図である。
【図３０】 本発明による超音波送受波器の第１３の実施形態の断面図である。
【図３１】 （ａ）および（ｂ）は、それそれ、本発明による超音波送受波器の第１４の実施形態の上面図である。
【図３２】 本発明による超音波送受波器の第１５の実施形態の断面図である。
【図３３】 本発明の実施形態１６における超音波流量計を示すブロック図である。
【図３４】 （ａ）および（ｂ）は、本発明の超音波流量計で測定される波形を示すグラフである。
【図３５】 従来の超音波流量計を示す断面図である。
【図３６】 従来の超音波送受信器の断面図である。 [Document Name] Description
【Technical field】
[0001]
The present invention relates to an ultrasonic transmitter / receiver having an acoustic matching layer, a manufacturing method thereof, and an ultrasonic flowmeter including the ultrasonic transmitter / receiver.
[Background]
[0002]
In recent years, ultrasonic flowmeters that determine the flow rate based on the moving speed by measuring the time during which the ultrasonic wave is transmitted for a predetermined distance in the pipe through which the fluid flows and measuring the moving speed of the fluid have been used in gas meters and the like. It's getting on.
[0003]
FIG. 35 shows a cross-sectional configuration of the main part of this type of ultrasonic flowmeter. The ultrasonic flowmeter is arranged so that a fluid to be measured whose flow rate is to be measured flows in the pipe. A pair of ultrasonic transmitters / receivers 101a and 101b are installed on the tube wall 102 so as to face each other. The ultrasonic transceivers 101a and 101b are configured by using a piezoelectric vibrator such as a piezoelectric ceramic as an electric energy / mechanical energy conversion element, and exhibit resonance characteristics like a piezoelectric buzzer and a piezoelectric oscillator.
[0004]
In the state shown in FIG. 35, the ultrasonic transceiver 101a is used as an ultrasonic transmitter, and the ultrasonic transceiver 101b is used as an ultrasonic receiver.
[0005]
When an AC voltage having a frequency near the resonance frequency of the ultrasonic transmitter / receiver 101a is applied to a piezoelectric body (piezoelectric vibrator) in the ultrasonic transmitter / receiver 101a, the ultrasonic transmitter / receiver 101a functions as an ultrasonic transmitter, Ultrasound is emitted into the fluid. The emitted ultrasonic wave propagates to the path L1 and reaches the ultrasonic transceiver 101b. At this time, the ultrasonic transmitter / receiver 101b functions as a receiver, receives the ultrasonic wave, and converts it into a voltage.
[0006]
Next, the ultrasonic transmitter / receiver 101b functions as an ultrasonic transmitter and the ultrasonic transmitter / receiver 101a functions as an ultrasonic receiver. That is, by applying an alternating voltage having a frequency near the resonance frequency of the ultrasonic transmitter / receiver 101b to the piezoelectric body in the ultrasonic transmitter / receiver 101b, ultrasonic waves are emitted from the ultrasonic transmitter / receiver 101b into the fluid. The emitted ultrasonic wave propagates along the path L2 and reaches the ultrasonic transceiver 101a. The ultrasonic transmitter / receiver 101a receives the propagated ultrasonic wave and converts it into a voltage.
[0007]
As described above, since the ultrasonic transmitters / receivers 101a and 101b alternately function as a transmitter and a receiver, they are generally collectively referred to as an ultrasonic transmitter / receiver (or an ultrasonic transmitter / receiver). .
[0008]
In the ultrasonic flow meter shown in FIG. 35, when an AC voltage is continuously applied, ultrasonic waves are continuously emitted from the ultrasonic transceiver and it becomes difficult to measure the propagation time. Is used as a drive voltage.
[0009]
Hereinafter, the measurement principle of the ultrasonic flowmeter will be described in more detail.
[0010]
When an ultrasonic burst signal is radiated from the ultrasonic transmitter / receiver 101a by applying a driving burst voltage signal to the ultrasonic transmitter / receiver 101a, the ultrasonic burst signal propagates through the path L1 and becomes ultrasonic transmitter / receiver after t time. 101b is reached. The distance of the route L1 is assumed to be L like the distance of the route L2.
[0011]
The ultrasonic transceiver 101b can convert only the transmitted ultrasonic burst signal into an electrical burst signal with a high S / N ratio. This electric burst signal is electrically amplified and applied again to the ultrasonic transceiver 101a to radiate the ultrasonic burst signal. A device that performs such an operation is called a “sing-around type device”. In addition, the time until the ultrasonic pulse reaches the ultrasonic transmitter / receiver 102b after the ultrasonic pulse is emitted from the ultrasonic transmitter / receiver 101a is referred to as a “sing-around period”. The reciprocal of “sing-around period” is called “sing-around frequency”.
[0012]
In FIG. 35, the flow velocity of the fluid flowing in the pipe is V, the velocity of the ultrasonic wave in the fluid is C, and the angle between the direction of flow of the fluid and the propagation direction of the ultrasonic pulse is θ. When the ultrasonic transmitter / receiver 101a is used as an ultrasonic transmitter / receiver and the ultrasonic transmitter / receiver 101b is used as an ultrasonic receiver, the time required for the ultrasonic pulse from the ultrasonic transmitter / receiver 101a to reach the ultrasonic transmitter / receiver 101b. Assuming that the sing-around period is t1 and the sing-around frequency f1, the following equation (1) is established.
[0013]
f1 = 1 / t1 = (C + Vcos θ) / L (1)
[0014]
On the contrary, if the ultrasonic transmitter / receiver 101b is used as an ultrasonic transmitter and the ultrasonic transmitter / receiver 101a is used as an ultrasonic receiver, t2 is a sing-around period and sing-around frequency f2 is The relationship (2) is established.
[0015]
f2 = 1 / t2 = (C−Vcos θ) / L (2)
[0016]
The frequency difference Δf between both sing-around frequencies is expressed by the following equation (3).
[0017]
Δf = f1-f2 = 2V cos θ / L (3)
[0018]
According to Expression (3), the flow velocity V of the fluid can be obtained from the distance L of the ultrasonic propagation path and the frequency difference Δf. The flow rate can be determined from the flow velocity V.
[0019]
Such an ultrasonic flowmeter requires high accuracy. In order to increase accuracy, the acoustic impedance of the acoustic matching layer formed on the ultrasonic transmission / reception surface of the piezoelectric body in the ultrasonic transmission / reception device is important. The acoustic matching layer plays an important role particularly when the ultrasonic transceiver radiates (transmits) an ultrasonic wave to a gas and receives an ultrasonic wave that has propagated through the gas.
[0020]
Hereinafter, the role of the acoustic matching layer will be described with reference to FIG. FIG. 36 shows a cross-sectional configuration of a conventional ultrasonic transceiver 103.
[0021]
The illustrated ultrasonic transmitter / receiver 103 includes a piezoelectric body 106 fixed inside the sensor case 105 and an acoustic matching layer 104 fixed outside the sensor case 105. The acoustic matching layer 104 is bonded to the sensor case 105 with an epoxy adhesive or the like. Similarly, the piezoelectric body 106 is also bonded to the sensor case 105.
[0022]
The ultrasonic vibration of the piezoelectric body 106 is transmitted to the sensor case 106 via the adhesive layer, and further transmitted to the acoustic matching layer 104 via another adhesive layer. Thereafter, the ultrasonic vibration is radiated as a sound wave to the gas (ultrasonic propagation medium) in contact with the acoustic matching layer 104.
[0023]
The role of the acoustic matching layer 104 is to efficiently propagate the vibration of the piezoelectric body to the gas. Hereinafter, this point will be described in more detail.
[0024]
The acoustic impedance Z of a substance is defined by the following equation (4) using the speed of sound C in the substance and the density ρ of the substance.
[0025]
Z = ρ × C (4)
[0026]
The acoustic impedance of the gas to be radiated by ultrasonic waves is greatly different from the acoustic impedance of the piezoelectric body. An acoustic impedance Z1 of a piezoelectric ceramic such as PZT (lead zirconate titanate) which is a general piezoelectric body is 30 × 10. ⁶ kg / m ² / S. On the other hand, the acoustic impedance Z3 of air is 400 kg / m. ² / S.
[0027]
Sound waves are likely to be reflected at the boundary surfaces of substances having different acoustic impedances, and the intensity of the sound waves transmitted through the boundary surfaces is reduced. For this reason, a substance having an acoustic impedance Z2 represented by Expression (5) is inserted between the piezoelectric body and the gas.
[0028]
Z2 = (Z1 × Z3) ^1/2 ... (5)
[0029]
When a substance having such an acoustic impedance Z2 is inserted, reflection at the boundary surface is suppressed, and sound wave transmittance is improved.
[0030]
Acoustic impedance Z1 is 30 × 10 ⁶ kg / m ² / S, acoustic impedance Z3 is 400 kg / m ² / S, the acoustic impedance Z2 satisfying the equation (5) is 11 × 10 ^Four kg / m ² / S. 11x10 ^Four kg / m ² A substance having a value of / s must naturally satisfy the equation (4), ie Z2 = ρ × C. It is extremely difficult to find such substances from solid materials. The reason is that it is required that the density ρ is sufficiently small and the sound velocity C is low while being solid.
[0031]
At present, as a material for the acoustic matching layer, a material obtained by solidifying a glass balloon or a plastic balloon with a resin material is widely used. Moreover, as a method for producing such a material suitable for the acoustic matching layer, for example, Japanese Patent No. 2559144 discloses a method of thermally compressing a hollow glass sphere and a method of foaming a molten material.
[0032]
However, the acoustic impedance of these materials is 50 × 10 ^Four kg / m ² It is a value larger than / s, and it is difficult to say that the expression (5) is satisfied. In order to obtain a highly sensitive ultrasonic transmitter / receiver, it is necessary to form an acoustic matching layer with a material having a further reduced acoustic impedance.
[0033]
In order to meet such a demand, the applicant of the present application has invented an acoustic matching material that sufficiently satisfies the expression (5), and disclosed it in the specification of Japanese Patent Application No. 2001-056051. This material is produced using a dry gel imparted with durability, has a low density ρ, and a low sound velocity C.
[0034]
An ultrasonic transmitter / receiver including an acoustic matching layer formed of a material such as a dry gel having an extremely low acoustic impedance can efficiently transmit / receive ultrasonic waves to / from a gas. As a result, an apparatus capable of measuring the gas flow rate with high accuracy is realized.
DISCLOSURE OF THE INVENTION
[Problems to be solved by the invention]
[0035]
However, materials with very low acoustic impedance, such as dry gels, generally have low mechanical strength. In particular, a dry gel is relatively strong against stress in the compression direction, but is extremely weak against stress in the tension and bending directions, and is easily broken by a weak impact.
[0036]
Moreover, since the speed of sound of such a material is very slow, the thickness of an appropriate acoustic matching layer (about ¼ of the transmission / reception wavelength) for obtaining the maximum transmission / reception sensitivity becomes very thin. For example, when the sound speed of the material is 60 to 400 m / s, when transmitting and receiving ultrasonic waves of about 500 kHz, the preferable thickness of the acoustic matching layer is about 30 to 200 μm. When it is thin, it is extremely difficult to handle the acoustic matching layer as a single member, and it is almost impossible to fabricate an ultrasonic transceiver by adhering the acoustic matching layer to a sensor case or a piezoelectric body. Even if it is possible, it is difficult to put it to practical use from the viewpoint of manufacturing yield and cost.
[0037]
Furthermore, due to the low mechanical strength of the acoustic matching layer, there is a possibility that the reliability of the acoustic matching layer may decrease due to the ultrasonic vibration itself inducing peeling of the acoustic matching layer during use as an ultrasonic transceiver.
[0038]
The present invention has been made in view of the above problems, and the object of the present invention is to provide an acoustic matching layer formed of a material having low mechanical strength such as a dry gel and a slow sound speed, and can be manufactured with high yield. Another object of the present invention is to provide a highly reliable ultrasonic transmitter / receiver and a method for manufacturing the same.
[0039]
Another object of the present invention is to provide an ultrasonic flowmeter provided with the above-described ultrasonic transceiver.
[Means for Solving the Problems]
[0040]
The ultrasonic transceiver according to the present invention includes a piezoelectric body, an acoustic matching layer provided on the piezoelectric body, and a position fixed to the piezoelectric body in contact with at least a part of a side surface of the acoustic matching layer. And a protective portion provided in the.
[0041]
In a preferred embodiment, the protection portion protrudes in an ultrasonic radiation direction from a plane at the same level as the main surface of the piezoelectric body, and the height of the protection portion with respect to the main surface of the piezoelectric body is the acoustic level. Specifies the thickness of the matching layer.
[0042]
In preferable embodiment, the said height of the said protection part is 5 micrometers or more and 2500 micrometers or less.
[0043]
In a preferred embodiment, the thickness of the acoustic matching layer is substantially equal to the height of the protective part.
[0044]
In a preferred embodiment, the thickness of the acoustic matching layer is about ¼ of the wavelength of the ultrasonic wave transmitted and / or received by the piezoelectric body.
[0045]
In a preferred embodiment, the acoustic matching layer has a density of 50 kg / m. ^Three 1000kg / m ^Three It is formed from the following materials.
[0046]
In a preferred embodiment, the acoustic matching layer has an acoustic impedance of 2.5 × 10 6. ^Three kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² / S or less.
[0047]
In a preferred embodiment, the acoustic matching layer is formed from an inorganic material.
[0048]
In a preferred embodiment, the inorganic material is an inorganic oxide dry gel.
[0049]
In a preferred embodiment, the inorganic oxide has a water-repellent solid skeleton.
[0050]
In a preferred embodiment, the acoustic matching layer is solidified from a fluid state on the piezoelectric body provided with the protective portion.
[0051]
In a preferred embodiment, it comprises a lower acoustic matching layer provided between the main surface of the piezoelectric body and the acoustic matching layer, and the protective part protrudes from the main surface of the lower acoustic matching layer, The height of the protective part with respect to the main surface of the acoustic matching layer defines the thickness of the acoustic matching layer located in the uppermost layer.
[0052]
In a preferred embodiment, the protection part is constituted by a part of the lower acoustic matching layer, and is integrated with the lower acoustic matching layer.
[0053]
In preferable embodiment, the said height of the said protection part is 5 micrometers or more and 2500 micrometers or less.
[0054]
In a preferred embodiment, the height of the protective part is substantially equal to the thickness of the acoustic matching layer located in the uppermost layer.
[0055]
In a preferred embodiment, each of the acoustic matching layer and the lower acoustic matching layer has a thickness of about ¼ of the wavelength of the ultrasonic wave transmitted and received by the piezoelectric body.
[0056]
In a preferred embodiment, the density of the sound matching layer is 50 kg / m. ^Three 1000kg / m ^Three It is as follows.
[0057]
In a preferred embodiment, the acoustic impedance of the lower acoustic matching layer is larger than the acoustic impedance of the acoustic matching layer, and is 2.5 × 10. ^Three kg / m ² / S or more 3.0 × 10 ⁷ kg / m ² / S or less.
[0058]
In a preferred embodiment, the protection part is present on the outer periphery of the acoustic matching layer located in the uppermost layer.
[0059]
In a preferred embodiment, the protection part covers the entire outer peripheral side surface of the acoustic matching layer located in the uppermost layer.
[0060]
In a preferred embodiment, the protection portion is disposed outside the main surface of the piezoelectric body.
[0061]
In a preferred embodiment, the protection part is provided on the main surface of the piezoelectric body.
[0062]
In a preferred embodiment, the protection part is provided on the lower acoustic matching layer.
[0063]
In a preferred embodiment, the protection part is constituted by a part of the lower acoustic matching layer, and is integrated with the lower acoustic matching layer.
[0064]
In a preferred embodiment, a structural support for supporting the piezoelectric body is further provided.
[0065]
In a preferred embodiment, a structural support for supporting the piezoelectric body is further provided, and the protection portion is provided on the structural support.
[0066]
In a preferred embodiment, the structural support is made of press-molded metal, and the protection part is constituted by a portion bent by press-forming the structural support.
[0067]
In preferable embodiment, the said height of the said protection part is 5 micrometers or more and 2500 micrometers or less.
[0068]
In a preferred embodiment, the thickness of the acoustic matching layer is substantially equal to the height of the protective part.
[0069]
In a preferred embodiment, the thickness of the acoustic matching layer is about ¼ of the wavelength of ultrasonic waves transmitted and received by the piezoelectric body.
[0070]
In a preferred embodiment, the density of the acoustic matching layer is 50 kg / m. ^Three 1000kg / m ^Three It is as follows.
[0071]
The acoustic impedance of the acoustic matching layer is 2.5 × 10 ^Three kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² / S or less.
[0072]
In a preferred embodiment, a back load material disposed on the back surface of the piezoelectric body is further provided, and the protection member is provided on the back load material.
[0073]
In a preferred embodiment, the protection part is constituted by a part of the back load material and is integrated with the back load material.
[0074]
In a preferred embodiment, at least a part of the surface of the acoustic matching layer that contacts the holding portion is subjected to a surface treatment for imparting a hydroxyl group.
[0075]
In a preferred embodiment, at least a part of the surface where the acoustic matching layer contacts the holding portion is subjected to a roughening treatment.
[0076]
In a preferred embodiment, at least a part of a surface where the acoustic matching layer is in contact with the holding portion is porous.
[0077]
In a preferred embodiment, a portion of the acoustic matching layer penetrates and is integrated with a portion of the ultrasonic transceiver that is in contact with the acoustic matching layer.
[0078]
Another ultrasonic transceiver of the present invention includes a piezoelectric body that performs ultrasonic vibration, and a density of 50 kg / m. ^Three 1000kg / m ^Three And the acoustic impedance is 2.5 × 10 ^Three kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² Supporting an upper acoustic matching layer formed of a material of / s or less, a lower acoustic matching layer provided between the piezoelectric body and the upper acoustic matching layer, the lower acoustic matching layer and the piezoelectric body, An ultrasonic transmitter / receiver including a structural support that shields the piezoelectric body from an ultrasonic wave propagation fluid, and includes a protection unit that contacts at least a part of a side surface of the upper acoustic matching layer.
[0079]
The protection part is formed by a part of the lower acoustic matching layer, and is integrated with the lower acoustic matching layer.
[0080]
In a preferred embodiment, the elastic modulus of the protection part is substantially equal to the elastic modulus of the acoustic matching layer.
[0081]
The ultrasonic flowmeter of the present invention includes a flow rate measurement unit through which a fluid to be measured flows, a pair of ultrasonic transmitters / receivers that are provided in the flow rate measurement unit and transmits / receives ultrasonic signals, and the pair of ultrasonic transmitters / receivers. An ultrasonic flowmeter comprising: a measuring means for measuring a time during which the ultrasonic wave propagates; and a flow rate calculating means for calculating a flow rate based on a signal from the measuring means, wherein the pair of ultrasonic transceivers Each is one of the above-described ultrasonic transceivers.
[0082]
In a preferred embodiment, the piezoelectric body of the ultrasonic transceiver is shielded from the fluid to be measured.
[0083]
The apparatus of the present invention includes any one of the above-described ultrasonic transceivers.
[0084]
An ultrasonic transceiver manufacturing method according to the present invention includes a step (a) of preparing a piezoelectric body having a main surface and a convex portion received on the main surface, and acoustic matching on the main surface of the piezoelectric body. Forming a layer and contacting at least part of the side surface of the acoustic matching layer with the side surface of the convex portion.
[0085]
In a preferred embodiment, the step (b) includes a step of supplying a gel raw material onto the main surface of the piezoelectric element, a step of forming the acoustic matching layer by drying and solidifying the gel raw material,
Is included.
[0086]
In a preferred embodiment, the step (a) includes a step of processing the surface of the piezoelectric body to form the main surface and the convex portion.
[0087]
In a preferred embodiment, the step (a) includes a step of fixing the convex portion on the surface of the piezoelectric body.
[0088]
In a preferred embodiment, the step (a) includes a step of fixing the piezoelectric body to the structural support.
[0089]
The method of manufacturing an ultrasonic transceiver according to the present invention includes an ultrasonic transceiver including an upper acoustic matching layer, a piezoelectric body, and a lower acoustic matching layer provided between the upper acoustic matching layer and the piezoelectric body. A method of manufacturing, comprising a step of preparing a member having a recess and functioning as the lower acoustic matching layer, and a step of supplying a gel raw material to the recess of the member (b), And (c) forming the upper acoustic matching layer by drying and solidifying the gel raw material.
[0090]
In a preferred embodiment, the step (b) includes a step of allowing the gel material to penetrate into the member.
[0091]
In a preferred embodiment, the gel raw material is allowed to penetrate the entire member.
[0092]
In a preferred embodiment, the step (b) is performed after fixing the positional relationship between the member and the piezoelectric body. The step (b) may be performed before fixing the positional relationship between the member and the piezoelectric body.
[0093]
The ultrasonic transmitter / receiver of the present invention includes a piezoelectric body, an acoustic matching layer provided on the piezoelectric body, and a protection unit disposed so as to be in contact with the outer peripheral surface of the acoustic matching layer.
[0094]
The ultrasonic transmitter / receiver of the present invention is disposed so as to be in contact with a structural support, a piezoelectric body and an acoustic matching layer provided at positions facing each other with the structural support interposed therebetween, and an outer peripheral surface of the acoustic matching layer. And a protection unit.
[0095]
Still another ultrasonic transducer according to the present invention includes a piezoelectric body having a main surface for transmitting and / or receiving ultrasonic waves, and an acoustic matching member provided on the main surface of the piezoelectric body. In the ultrasonic transducer, the acoustic matching member includes a first acoustic matching portion and a second acoustic matching portion having an average density lower than an average density of the first acoustic matching portion. The first acoustic matching portion is in contact with a side surface of the second acoustic matching portion.
[0096]
In a preferred embodiment, the first acoustic matching portion is thicker than the second acoustic matching portion, is radiated from the main surface of the piezoelectric body, passes through the second acoustic matching portion, and is an upper surface of the first acoustic matching portion. The phase of the ultrasonic wave that has reached the same level position substantially coincides with the phase of the ultrasonic wave that has been radiated from the main surface and transmitted through the first acoustic matching portion and reached the upper surface of the first acoustic matching portion. ing
In a preferred embodiment, when the wavelength of the ultrasonic wave in the first acoustic matching portion is λ1, the thickness of the first acoustic matching portion is k1 · λ1 (k1 is 1/8 or more and 1/3 or less). And the thickness of the twenty-first acoustic matching portion is different.
[0097]
In a preferred embodiment, the second acoustic matching portion includes N acoustic matching layers (N is an integer equal to or greater than 1), and each of the N acoustic matching layers includes the super acoustic layer in each acoustic matching layer. It has a magnitude of k2 times the wavelength of the sound wave (k2 is 1/8 or more and 1/3 or less).
[0098]
In a preferred embodiment, the thickness of the acoustic matching layer located on the outermost layer of the second acoustic matching portion is about 1/4 of the wavelength of the ultrasonic wave in the acoustic matching layer located on the outermost layer of the second acoustic matching portion. It is.
[0099]
In a preferred embodiment, the acoustic matching layer formed at a position closest to the main surface of the piezoelectric body in the second acoustic matching portion is made of the same material as the material of the first acoustic matching portion. .
[0100]
In a preferred embodiment, an acoustic matching layer formed at a position closest to the main surface of the piezoelectric body in the second acoustic matching portion is formed integrally with the first acoustic matching portion.
[0101]
In a preferred embodiment, at least one acoustic matching layer included in the second acoustic matching portion is formed of a dry gel.
[0102]
In a preferred embodiment, the dry gel is made of an inorganic material.
[0103]
In a preferred embodiment, the dry gel has a water-repellent solid skeleton.
[0104]
In a preferred embodiment, at least a part of a surface in contact with the acoustic matching portion among the members constituting the acoustic wave transmitter / receiver is a rough surface or a porous surface.
[0105]
In a preferred embodiment, among the members constituting the ultrasonic transducer, at least part of the surface in contact with the second acoustic matching portion, a part of the second acoustic matching portion is infiltrated and integrated with the member. .
[0106]
In a preferred embodiment, at least a part of the second acoustic matching portion is made of a dry gel, and the first acoustic matching portion is made of a material having higher mechanical strength than the dry gel.
[0107]
In a preferred embodiment, at least a part of the first acoustic matching portion is made of porous ceramics.
[0108]
In a preferred embodiment, the thickness of the first acoustic matching portion changes according to the position on the main surface of the piezoelectric body.
[0109]
In a preferred embodiment, the thickness of the second acoustic matching portion changes according to the position on the main surface of the piezoelectric body.
[0110]
The ultrasonic flowmeter of the present invention includes a flow rate measurement unit through which a fluid to be measured flows, a pair of ultrasonic transducers that are provided in the flow rate measurement unit and transmit / receive ultrasonic signals, and the pair of ultrasonic transmission / reception units. An ultrasonic flowmeter comprising: a measuring unit that measures a time during which an ultrasonic wave propagates between devices; and a flow rate calculating unit that calculates a flow rate based on a signal from the measuring unit, wherein the pair of ultrasonic waves Each of the transducers is one of the above-described ultrasonic transducers.
[0111]
In a preferred embodiment, the piezoelectric body of the ultrasonic transducer is shielded from the fluid to be measured.
[0112]
In a preferred embodiment, the fluid to be measured is a gas.
[0113]
The apparatus of the present invention includes any one of the above ultrasonic transducers.
[0114]
The method for manufacturing an ultrasonic transducer according to the present invention includes (a) a first surface and a second surface opposite to the first surface, and electrodes are provided on the first and second surfaces. Preparing a formed piezoelectric body; (b) forming a second acoustic matching portion on at least one side of the first and second surfaces of the piezoelectric body; and (c) the piezoelectric body. Supplying a gel raw material into the space formed by the second acoustic matching portion, (d) obtaining a wet gel by gelling the gel raw material liquid, and (e) obtaining the wet gel Drying.
[0115]
In a preferred embodiment, the step (c) includes: (c1) supplying a first gel raw material into the space; and (c2) gelling the first gel raw material liquid to form a first wet gel. Forming a layer; (c3) supplying a second gel raw material on the first wet gel layer; and (c4) gelling the second gel raw material liquid to form a second wet. Forming a gel layer, wherein the step (e) includes first and second wet gel layers, respectively, by drying the first and second wet gel layers. Forming a matching layer and a second acoustic matching layer.
[0116]
In a preferred embodiment, in the step (c4), the first wet gel layer is modified so as to change an acoustic impedance of the first acoustic matching layer.
[0117]
An ultrasonic transducer according to the present invention includes an ultrasonic wave including a piezoelectric body having a main surface for transmitting and / or receiving ultrasonic waves, and an acoustic matching member provided on the main surface of the piezoelectric body. In the transducer, the acoustic matching member includes a first acoustic matching portion and a second acoustic matching portion having a mechanical strength lower than the mechanical strength of the first acoustic matching portion, The first acoustic matching portion is in contact with the side surface of the second acoustic matching portion.
【The invention's effect】
According to the present invention, it is possible to put to practical use a high-performance ultrasonic transceiver that uses a thin acoustic matching layer formed of a material having extremely low acoustic impedance and low mechanical strength. Reliability in use is also improved. In the first aspect of the present invention, since the acoustic matching layer is protected and a protective portion that can also be used for defining the thickness of the acoustic matching layer is provided, the mechanical strength is low and the thin acoustic matching layer is provided. Can be formed with high accuracy and good reproducibility. As a result, a highly reliable ultrasonic transducer that can transmit and receive broadband ultrasonic waves with high sensitivity is provided. Further, in the second aspect of the present invention, a protective part (protective part / acoustic matching layer = protective matching layer) that also functions as an acoustic matching layer can be provided at an arbitrary position in the main surface of the piezoelectric body. By adjusting the sound speed and thickness of the two types of acoustic matching layers, it is possible to align the phases of the ultrasonic waves radiated from the two types of acoustic matching layers having different thicknesses and increase the transmission / reception sensitivity of the ultrasonic waves. .
BEST MODE FOR CARRYING OUT THE INVENTION
[0118]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0119]
(Embodiment 1)
FIG. 1 shows a section of an ultrasonic transceiver (ultrasonic transducer) in the first embodiment of the present invention. The illustrated ultrasound transmitter / receiver 1 includes a piezoelectric body 4, an acoustic matching layer 3 provided on the piezoelectric body 4, and a protection unit 2 fixed to the piezoelectric body 4.
[0120]
The piezoelectric body 4 is made of a piezoelectric material and is polarized in the thickness direction. Electrodes (not shown) are formed on the upper and lower surfaces of the piezoelectric body 4 and emit ultrasonic waves based on signals applied to the electrodes. When receiving ultrasonic waves, a voltage signal is generated between the electrodes. In the present invention, the material of the piezoelectric body 4 is arbitrary, and a known material can be used.
[0121]
The height H of the protective portion 2 with respect to the main surface (ultrasonic wave transmitting / receiving surface) S1 of the piezoelectric body 4 defines the thickness of the acoustic matching layer 3. In a preferred embodiment, the height of the protective portion 2 is acoustic. It is substantially equal to the thickness of the matching phase layer 3.
[0122]
FIG. 2 shows the top surface of the ultrasonic transceiver 1 of FIG. As can be seen from FIG. 2, in the ultrasonic transceiver 1 of the present embodiment, the ring-shaped protection unit 2 surrounds the acoustic matching layer 3, and the entire outer peripheral surface (side surface) of the acoustic matching layer 3 is within the protection unit 2. It is in contact with the peripheral surface. By providing such a protective portion 2 on the upper surface of the piezoelectric body 4, the acoustic matching layer 3 is hardly peeled off from the piezoelectric body 4, and damage to the acoustic matching layer 3 can be prevented. As a result, the reliability of the ultrasonic transceiver 1 in the manufacturing stage and in the use stage is significantly improved.
[0123]
In addition, according to the manufacturing method described later, the thickness of the acoustic matching layer 3 can be controlled with high accuracy by adjusting the height H of the protective portion 2. Therefore, since the acoustic matching layer 3 can be stably formed with high accuracy, an ultrasonic transmitter / receiver excellent in quality can be manufactured with high yield. If the thickness of the acoustic matching layer 3 varies from element to element, the characteristics (sensitivity, etc.) of the ultrasonic transmitter / receiver will vary, so it is important to form the acoustic matching layer 3 having a predetermined thickness with good reproducibility. . As described above, an appropriate acoustic matching layer thickness for obtaining the maximum transmission / reception sensitivity is about ¼ of the wavelength of ultrasonic waves to be transmitted / received. For this reason, when transmitting and receiving ultrasonic waves of about 500 kHz using a dry gel with a sound velocity of about 280 m / s as the acoustic matching layer, it is necessary to set the preferred thickness of the acoustic matching layer of the dry gel to about 140 μm. . If the thickness differs by about 10%, the transmission / reception sensitivity may vary by about 20%. As described above, the transmission / reception sensitivity greatly fluctuates only by a slight change in the thickness of the acoustic matching layer 3, but according to the present embodiment, the acoustic matching layer 3 having a desired thickness is formed with good reproducibility. Yes.
[0124]
The ultrasonic transceiver 1 of this embodiment is manufactured as follows, for example.
[0125]
First, a piezoelectric body 4 is prepared according to the wavelength of ultrasonic waves to be transmitted / received. As the piezoelectric body 4, a highly piezoelectric material such as piezoelectric ceramics or a piezoelectric single crystal is preferable. As the piezoelectric ceramic, lead zirconate titanate, barium titanate, lead titanate, lead niobate, or the like can be used. As the piezoelectric single crystal, lead zirconate titanate single crystal, lithium niobate, quartz, or the like can be used.
[0126]
In this embodiment, lead zirconate titanate piezoelectric ceramics are used as the piezoelectric body 4, and the frequency of ultrasonic waves to be transmitted and received is set to 500 kHz. In order to allow the piezoelectric body 4 to efficiently transmit and receive such ultrasonic waves, the resonance frequency of the element is designed to be 500 kHz. For this reason, in this embodiment, the piezoelectric body 4 formed from a piezoelectric ceramic having a cylindrical shape with a diameter of 12 mm and a thickness of about 3 mm is used.
[0127]
A ring-shaped protective portion 2 having an outer diameter of 12 mm, an inner diameter of 11 mm, and a thickness of 140 μm is bonded to such a piezoelectric body 4. In the present embodiment, a stainless metal ring is used as the protection unit 2. The stainless steel protective part 2 and the piezoelectric body 4 can be joined by bonding with an adhesive. For example, an epoxy resin may be used as an adhesive, and left for 2 hours in a thermostatic bath at 150 ° C. while applying a pressure of 0.2 MPa to be cured.
[0128]
In the present embodiment, the acoustic matching layer 3 is formed from a dried gel. Since the acoustic velocity of the acoustic matching layer 3 formed from the dried gel is about 280 m / s, the wavelength of the ultrasonic wave in the acoustic matching layer 3 is about 640 μm. For this reason, the thickness of the acoustic matching layer 3 is set to 140 μm so as to be equal to about ¼ of the ultrasonic wavelength in the acoustic matching layer 3. In order to form the acoustic matching layer 3 having this thickness, in the present embodiment, the thickness of the protective portion 2 is set to 140 μm.
[0129]
The role of the protection unit 2 is primarily to protect the acoustic matching layer 3 from external mechanical shocks or thermal shocks during the manufacturing and use stages of the ultrasonic transceiver 1. Secondly, it is also important to protect the ultrasonic transmitter / receiver 1 from vibration of ultrasonic waves to be transmitted / received during operation (use) as the ultrasonic transmitter / receiver 1.
[0130]
In order for the acoustic matching layer 3 to fulfill its role, it is extremely important that the piezoelectric body 4 and the acoustic matching layer 3 are in close contact with each other. If even a slight separation occurs between the piezoelectric body 4 and the acoustic matching layer 3, it cannot function as the acoustic matching layer 3.
[0131]
In order to maintain the adhesion between the piezoelectric body 4 and the acoustic matching layer 3, the inventor of the present application has a very effective structure in which the protective portion 2 is provided on the outer peripheral portion of the acoustic matching layer 3 as shown in FIG. I found something. When the protection unit 2 is not provided, there is a possibility that the characteristic deterioration is greatly progressed when the ultrasonic transmitter / receiver 1 is manufactured or used, and the high-performance ultrasonic transmitter / receiver 1 cannot be put into practical use.
[0132]
The acoustic matching layer 3 of the present embodiment is formed of a material having an extremely small acoustic impedance defined by the product of the density ρ and the speed of sound C (ρ × C). For this reason, the transmission / reception efficiency of ultrasonic waves with respect to a gas such as air can be extremely increased. In the present embodiment, as described above, a dry gel is used as a material having an extremely low acoustic impedance.
[0133]
By forming the acoustic matching layer 3 from a dry gel, the acoustic matching between the gas and the piezoelectric body is improved compared to the case where the acoustic matching layer is formed from a conventional material in which a glass balloon or a plastic balloon is solidified with a resin material. Since it becomes very good, ultrasonic transmission / reception efficiency can be remarkably improved.
[0134]
The “dry gel” in the present specification is a porous body formed by a sol-gel reaction, and the solid skeleton portion solidified by the reaction of the gel raw material liquid passes through a wet gel composed of a solvent, It was formed by drying and removing the solvent. This dry gel is a nanoporous body in which continuous pores having an average pore diameter of several nanometers to several micrometers are formed by a nanometer-sized solid skeleton.
[0135]
Since the porous body has such a fine structure, the sound velocity propagating through the solid portion becomes extremely small, and the sound velocity propagating through the gas portion inside the porous body due to the pores is extremely small. . Therefore, a very slow value of about 500 m / s or less is shown as the sound speed, and a low acoustic impedance completely different from that of the conventional acoustic matching layer can be obtained. In addition, since the nanometer-sized pores have a large pressure loss of gas, they have a feature that sound waves can be emitted with a high sound pressure when used as an acoustic matching layer.
[0136]
As a material for such a dry gel, various materials such as an inorganic material and an organic polymer material can be used. As the solid skeleton portion of the inorganic material, silicon oxide (silica), aluminum oxide (alumina), titanium oxide, or the like can be used. As the solid skeleton portion of the organic material, a general thermosetting resin or a thermoplastic resin can be used. For example, polyurethane, polyurea, phenol curable resin, polyacrylamide, polymethyl methacrylate, or the like can be used. .
[0137]
In the present embodiment, the acoustic matching layer 3 is formed from the above-described dry gel in the concave space P1 (see FIG. 1) formed in advance by the piezoelectric body 4 and the protection unit 2. That is, after the liquid gel material liquid is poured into the concave space P1 composed of the piezoelectric body 4 and the protection portion 2, gelation, hydrophobization, and drying are performed, so that the acoustic matching layer 3 is dried. Form a gel. In the present embodiment, a dry gel having a silicon oxide solid skeleton is used as the acoustic matching layer 3.
[0138]
Specifically, the acoustic matching layer 3 can be formed by sequentially performing the following steps 1 to 4.
[0139]
1. A gel raw material liquid (sol) prepared by preparing tetraethoxysilane, ethanol, and an aqueous ammonia solution (0.1 N) in a molar ratio of 1: 3: 4 is prepared.
[0140]
2. This gel raw material liquid is dropped into a concave space formed by the piezoelectric body and the protective portion with a dropper. At that time, an excessive amount of the gel raw material liquid is dropped from the volume of the concave space P1. Next, a grinding operation using a Teflon (registered trademark) flat plate (not shown) was performed so that the height of the gel raw material liquid accumulated in the concave space P1 was the same as the height H of the protective part. Then, cover with a Teflon plate.
[0141]
3. Leave at room temperature for about 1 day, and after the raw material solution has gelled (form wet gel), remove the Teflon plate. Thereafter, a hydrophobization treatment is performed in a 5% by weight hexane solution of trimethylethoxysilane.
[0142]
4. It introduce | transduces into a supercritical drying tank, and supercritical drying is performed on the conditions of 12 Mpa and 50 degreeC under a carbon dioxide atmosphere. A dry gel is thus formed.
[0143]
By such steps 1 to 4, for example, the density ρ is 0.3 × 10 ^Three kg / m ^Three The acoustic matching layer 4 having a sound velocity C of 280 m / s and a thickness of 140 μm can be formed.
[0144]
The present invention has a density of 50 kg / m ^Three 1000kg / m ^Three And the acoustic impedance is 2.5 × 10 ^Three kg / m ² / S or more 1.0 × 10 ⁶ kg / m ² When the acoustic matching layer is formed from a material of / s or less, a remarkable effect is exhibited. However, according to the above method, such an acoustic matching layer can be preferably produced.
[0145]
According to the above method, since the thickness of the acoustic matching layer 3 can be made substantially equal to the height H of the protective part 2, the thickness of the acoustic matching layer 3 can be controlled with high precision by the protective part 2. It can be said that the protection part 2 functions as a guide for the gel raw material liquid in a certain stage of the manufacturing process.
[0146]
According to the above method, since the acoustic matching layer 3 with little thickness variation can be formed with a high yield, it is possible to suppress variations in characteristics of the ultrasonic transceiver. In the present invention, it is not essential to make the height of the protective portion 2 equal to the thickness of the acoustic matching layer 3. When the height of the protection part 2 is larger than the thickness of the acoustic matching layer 3, the function of suppressing the contraction of the acoustic matching layer 3 and protecting it from mechanical impact is sufficiently exhibited. Conversely, even if the height of the protective portion 2 is smaller than the thickness of the acoustic matching layer 3, if the protective portion 2 is not provided, the acoustic matching layer 3 can be prevented from contracting and mechanical shock can be prevented. The function to protect is exhibited to a high degree.
[0147]
The transmission / reception waveform of the ultrasonic transmitter / receiver including the acoustic matching layer 3 manufactured by the above method was measured. A waveform diagram obtained by the measurement is shown in FIG. For comparison, FIG. 3B shows a transmission / reception waveform when a material in which a glass balloon is hardened with epoxy is used for an acoustic matching layer. The acoustic matching layer of the glass balloon used here has a density of 0.52 g / cm. ^Three The sound speed is 2500 m / s and the thickness is 1.25 mm.
[0148]
As described in the related art, it is desirable that the acoustic impedance of the acoustic matching layer has a value defined by Equation (5). In this embodiment, lead zirconate titanate piezoelectric ceramics are used for the piezoelectric body 4 and air is set as a propagation medium for propagating ultrasonic waves. Therefore, the density of the piezoelectric body 4 is 7.7 × 10. ^Three kg / m ^Three Since the sound speed is 3800 m / s, the acoustic impedance is about 29 × 10 ⁶ kg / m ² / S. On the other hand, the density of air is 0.00118 kg / m ^Three Since the sound speed is 340 m / s, the acoustic impedance is about 0.0004 × 10 ⁶ kg / m ² / S. For this reason, from the equation (5), the preferable acoustic impedance of the acoustic matching layer is theoretically about 0.1 × 10 10. ⁶ kg / m ² / S.
[0149]
The acoustic impedance of the acoustic matching layer 3 in the ultrasonic transceiver 1 of the present embodiment has a density of 0.3 × 10. ^Three kg / m ^Three Since the sound speed is 280 m / s, about 0.084 × 10 ⁶ kg / m ² / S, which is very close to the theoretical ideal value.
[0150]
As can be seen from FIG. 3, according to this embodiment, it is possible to obtain a transmission / reception sensitivity that is three times or more that of a conventional sensor. Moreover, in this embodiment, since the protective part 2 is provided, even an ultrasonic transmitter / receiver having an acoustic matching layer formed from a dry gel that has low mechanical strength and is fragile can be manufactured with high yield, and even in use. It can continue to operate reliably for a long time. An external vibration test, a thermal shock test, a continuous vibration test, etc. were performed to evaluate whether or not the acoustic matching layer 3 peels from the piezoelectric body 4 under severe conditions. However, the performance of the ultrasonic transmitter / receiver deteriorates. There was no, and very stable operation was confirmed.
[0151]
In the present embodiment, when the wet gel is dried to obtain a dry gel in the step 4, the supercritical drying method is used. However, drying in a normal atmosphere may be performed. In this case, contraction occurs in the process of changing from a wet gel to a dry gel, and a volume change of about 10 to 20% occurs. When such volume shrinkage occurs, the acoustic matching layer 3 peels from the piezoelectric body 4 in the conventional configuration. However, in the case of the present embodiment, as shown in FIG. 4, since the protective part 2 exists, the shrinkage of the dried gel mainly occurs only in the thickness direction. That is, almost no stress in the in-plane direction is generated at the interface between the piezoelectric body 4 and the acoustic matching layer 3, and peeling of the acoustic matching layer 3 is effectively prevented. Therefore, even if an air drying method that is simpler than the supercritical drying method is employed, the ultrasonic transceiver 1 with high sensitivity and high reliability can be manufactured, and the manufacturing cost can be reduced.
[0152]
In addition, it is preferable that the height of the protection part 2 is set so that the final thickness of the acoustic matching layer 3 has an optimum size in consideration of the shrinkage rate of the dried gel. Note that it is not preferable that the degree of shrinkage becomes excessively large and the thickness of the thinnest portion of the acoustic matching layer 3 decreases to 90% or less of the average thickness, because the characteristics of the acoustic matching layer 3 deteriorate. According to the supercritical drying method, the thickness of the thinnest portion of the acoustic matching layer 3 can be maintained at 98% or more of the average thickness.
[0153]
Surface treatments such as plasma cleaning and acid treatment are performed in advance on the upper surface of the piezoelectric body 4 in contact with the lower surface of the acoustic matching layer 3 and the inner surface of the protective portion 2 in contact with the side surface of the acoustic matching layer 3. Is preferred. When a hydroxyl group is formed on the contact surface by such treatment, the chemical bond between the dried gel, the piezoelectric body 4 and the protective portion 2 can be further strengthened.
[0154]
In order to realize a strong coupling between the acoustic matching layer 3 and the piezoelectric body 4 and the protection unit 2, a region in contact with the acoustic matching layer 3 may be roughened on the surface of the piezoelectric body 4 and the protection unit 2. . As a roughening method, normal file, blasting, physical or chemical etching, etc. can be used effectively.
[0155]
In order to improve the adhesion between the acoustic matching layer 3 and the protection part 2, it is also effective to use a porous material as the material of the protection part 2. By forming the protection part 2 from the porous body, a part of the acoustic matching layer 3 penetrates into the protection part 2 and is integrated, so that a stronger adhesion state can be obtained.
[0156]
Examples of the porous body that can be used for the protective part 2 include metals, ceramics, and resins manufactured by a foaming method or the like. Various materials such as stainless steel, nickel, and copper can be used as the porous metal, alumina and barium titanate can be used as the ceramic, and epoxy and urethane can be used as the resin.
[0157]
In this specification, “protecting the acoustic matching layer” not only protects the acoustic matching layer from mechanical vibrations and shocks, but also creates a sound matching layer from a material that shrinks during formation. It also includes suppressing the peeling of the matching layer. By adopting a member that protects the acoustic matching layer in this way, even if the acoustic matching layer is formed from a material having low mechanical strength and shrinkage, the function of the acoustic matching layer (piezoelectric It is possible to maintain a practical level of operation that can efficiently transmit and receive ultrasonic waves between the body and the ultrasonic propagation medium.
[0158]
(Embodiment 2)
A second embodiment of the present invention will be described with reference to FIG.
[0159]
In the present embodiment, the protection part and the piezoelectric body are integrated. Specifically, a recess 5a is formed in the central portion of the main surface of the piezoelectric body 5, and a part 5b of the piezoelectric body 5 is used as a protective portion. In other words, a part 5b of the piezoelectric body 5 functions as a protection portion, and the protection portion and the piezoelectric body are integrally formed.
[0160]
The ultrasonic transceiver of FIG. 5 is manufactured as follows.
[0161]
First, after preparing a piezoelectric body 5 that has been subjected to polarization treatment, one surface (main surface) of the piezoelectric body 5 is processed to form a recess 5a. The processing for forming the recess 5a can be performed by an end mill or sand blasting. The depth of the recess 5a corresponds to the height of the protection part (5b). Thereafter, an electrode is formed on the surface where the recess 5a is formed, and an electrode is also formed on the surface opposite to the recess of the piezoelectric body. The electrode is produced by forming a metal film such as gold or nickel by plating or sputtering, for example.
[0162]
According to the present embodiment, since the piezoelectric body 5 is processed and the peripheral portion of the main surface functions as a protective portion, a step of joining a separately manufactured protective portion to the piezoelectric body becomes unnecessary. When bonding is used for bonding of the protective part, it is necessary to consider the change in the height of the protective part due to the presence of the adhesive layer, but according to this embodiment, the protective part whose height is defined with high accuracy Therefore, the thickness of the acoustic matching layer 3 can be adjusted with high accuracy, and a high-performance ultrasonic transceiver can be provided stably.
[0163]
Also in this embodiment, it is preferable to perform a process of forming a hydroxyl group on a portion of the surface of the piezoelectric body 5 that is in contact with the acoustic matching layer 3. Further, when the piezoelectric body 5 is roughened during the process of forming the recess 5 a in the piezoelectric body 5, the adhesion between the acoustic matching layer 3 and the piezoelectric body 5 can be further improved.
[0164]
In the first embodiment and the second embodiment, the portion functioning as the protection portion is formed in a ring shape having a side surface perpendicular to the main surface of the piezoelectric body 5. However, the side surface of the protection portion may have a taper as shown in FIG. Moreover, as shown in FIG. 7, it is not necessary to be in contact with all of the outer peripheral side surface of the acoustic matching layer 3, and has a structure divided into a plurality of parts or a structure in which notches are formed in part. May be.
[0165]
According to the configuration of the first or second embodiment described above, even when a low-density material such as a dry gel and a slow sound speed is used for the acoustic matching layer, the protective unit strengthens the coupling between the acoustic matching layer and the piezoelectric body. Thus, it is possible to provide a high-performance ultrasonic transmission transducer with a high yield, while exhibiting high transmission / reception sensitivity and facilitating handling in the manufacturing process stage of the ultrasonic transmission transducer. In addition, an element having excellent reliability that is less likely to deteriorate due to mechanical shock at the stage of using the ultrasonic transducer and vibration caused by transmission / reception of ultrasonic waves is realized.
[0166]
(Embodiment 3)
A third embodiment of the present invention will be described with reference to FIG.
[0167]
A characteristic point of this embodiment is that the structure support 6 is provided. The structural support 6 shown in FIG. 8 includes a disc-like support portion 6a to which the acoustic matching layer 3 is fixed, and a cylindrical portion 6b extending continuously from the disc-like support portion in the axial direction. The end portion of the cylindrical portion is bent in an L shape in cross section, and is easily fixed to a shielding plate 60 or another device.
[0168]
The acoustic matching layer 3 and the protection part 2 are arranged on the surface of the support part 6a of the structural support body 6, and the piezoelectric body 4 is arranged on the back surface of the support part 6a. That is, the piezoelectric body 4 and the acoustic matching 3 are respectively provided at positions facing each other with the structure support 6 interposed therebetween. By using such a structure support body 6, handling of an ultrasonic transmitter / receiver (ultrasonic transmitter / receiver) becomes extremely easy.
[0169]
The structural support 6 can be constituted by a container (sensor case) that can be sealed. In this case, if the open end of the cylindrical portion 6b of the structural support 6 is closed with a shielding plate 60 or the like, and the inside of the structural support 6 is filled with an inert gas, the piezoelectric body 4 is removed from the fluid to be measured for flow velocity. Can be cut off. Since a voltage is applied to the piezoelectric body 4, if the piezoelectric body 4 is surrounded by a combustible gas, there is a risk of igniting the combustible gas. However, since the structural support 6 is composed of a sealed container and the inside is shut off from the outside, such an ignition can be prevented, so that ultrasonic waves can be safely emitted even to a combustible gas. . Even if the external gas is not flammable, the piezoelectric body 4 is shielded from the external gas even when the ultrasonic wave is emitted to the gas that may react with the piezoelectric body 4 and possibly deteriorate the characteristics of the piezoelectric body 4. By doing so, it is possible to suppress the deterioration of the piezoelectric body 4 and realize a highly reliable operation over a long period of time.
[0170]
In the example of FIG. 8, the protection unit 2 is disposed on the outer peripheral portion of the ultrasonic transmission / reception surface of the piezoelectric body 4. In general, the protection unit 2 does not play the role of the acoustic matching layer 3. Therefore, when the protection unit 2 is disposed on the main surface of the piezoelectric body 4, the portion does not contribute to transmission / reception of ultrasonic waves, and transmission / reception sensitivity decreases. End up.
[0171]
In order to prevent the structural support body 6 from being an acoustic obstruction factor, it is desirable that the thickness of the disk-shaped support portion 6a with which the piezoelectric body 4 contacts is 1/8 or less of the wavelength of ultrasonic waves to be transmitted and received. . By making this thickness about 1/8 or less of the wavelength, the structural support 6 does not hinder the propagation of ultrasonic waves.
[0172]
In the present embodiment, stainless steel is used as the material of the structural support 6 and the thickness of the structural support 6 is set to 0.2 mm. Since the speed of sound in stainless steel is about 5500 m / s, 0.2 mm corresponds to about 1/55 of the wavelength in an ultrasonic wave having a frequency of 500 kHz. Since the structural support 6 is formed of such thin stainless steel, even if the structural support 6 is interposed in the ultrasonic wave propagation path, there is almost no acoustic obstacle.
[0173]
The material of the structural support 6 is not limited to a metal material, and a material according to the purpose can be selected from ceramic, glass, and resin. In the present embodiment, the external fluid and the piezoelectric body are reliably separated, and the strength that can prevent the contact between the piezoelectric body and the external fluid is given even if some mechanical shock is applied to the structural support 6. Therefore, the structural support 6 is produced from a metal material. Thereby, for example, even when ultrasonic waves are transmitted / received for flammable or explosive gases, high safety can be ensured.
[0174]
When ultrasonic waves are transmitted / received to / from a safe gas, the structural support 6 may be formed from a material such as a resin for the purpose of cost reduction.
[0175]
In order to improve the adhesion between the structural support 6 and the acoustic matching layer 3, plasma treatment or acid treatment for adding a hydroxyl group is performed in advance on the surface of the structural support 6 in contact with the acoustic matching layer 3. Is preferred. Further, this portion may be roughened by sanding, sandblasting, chemical and / or physical etching, or the like.
[0176]
(Embodiment 4)
Next, a fourth embodiment of the present invention will be described with reference to FIG.
[0177]
In the ultrasonic transceiver according to this embodiment, a part 7a of the structure support 7 functions as a protection part, and the structure support 7 and the protection part are integrated. For example, when the structural support 7 is produced by press molding a metal material such as stainless steel, the concave portion 7b is formed in the disk-shaped support portion, and the periphery of the concave portion 7b (bent by the press molding of the structural support 7 is formed. Part 7a) can be used as a protective part.
[0178]
By employ | adopting such a structure, the process of joining a protection part to a structure support body can be skipped. Further, similarly to the first embodiment, the height of the protective portion does not vary due to the adhesive layer, so that a highly sensitive ultrasonic transceiver can be manufactured with high yield.
[0179]
In addition, in FIG. 9, although the plate for sealing the structure support body 7 is not described, you may adhere or integrate such a plate to the structure support body 7 as needed. The same applies to other embodiments described below.
[0180]
(Embodiment 5)
A fifth embodiment of the present invention will be described with reference to FIGS. 10 and 11.
[0181]
The ultrasonic transmitter / receiver of this embodiment includes another acoustic matching layer (lower acoustic matching layer) 8 disposed between the acoustic matching layer 3 and the piezoelectric body 4. Except for the insertion of the lower acoustic matching layer 8, the configuration of the present embodiment is the same as the configuration of the second embodiment.
[0182]
The acoustic matching layer plays a role of suppressing the internal reflection of the sound wave due to the mismatch of the acoustic impedance and efficiently radiating the ultrasonic wave from the piezoelectric body to the medium (ultrasonic propagation medium). One layer is sufficient for such an acoustic matching layer when transmitting or receiving ultrasonic waves having a single frequency (continuous wave ultrasonic waves).
[0183]
In contrast, a normal ultrasonic transmitter / receiver transmits and receives pulsed or bursty ultrasonic waves. Pulsed or bursty ultrasonic waves contain broadband frequency components rather than a single frequency component. In order to transmit and receive such ultrasonic waves with high sensitivity, it is preferable that the acoustic impedance of the acoustic matching layer is gradually changed between the piezoelectric body and the ultrasonic propagation medium. In order to gradually change the acoustic impedance in this way, the acoustic matching layer may be multilayered and the acoustic impedance of the constituent layers may be gradually shifted.
[0184]
In the present embodiment, as shown in FIG. 10, the acoustic matching layer is made into two layers. Specifically, a porous sintered body made of ceramics is used as the lower acoustic matching layer 8. This acoustic matching layer 8 has an apparent density of about 0.64 × 10 6. ^Three kg / m ^Three , Sound speed is 2000m / s, acoustic impedance is about 1.28 × 10 ⁶ kg / m ² / S. As the ceramic, a barium titanate-based material is used.
[0185]
The “apparent density” is a density including a space portion included in the porous body. In the porous ceramics, about 80% of the volume is a space part (hole), and the solid part of the ceramic is about 20% by volume of the whole. Such porous ceramics are formed by mixing and press-molding resin balls and ceramic powder, and then heating and burning away the resin balls in the process of sintering the ceramics. If heating is performed rapidly during sintering, the resin balls expand or rapidly gasify, and the ceramic structure is destroyed. Therefore, it is preferable to perform gentle heating.
[0186]
In the present embodiment, such a lower acoustic matching layer 8 is fixed to the piezoelectric body 4 (directly the structural support 6), and then the acoustic matching layer 8 on the side opposite to the piezoelectric body 4. The protective part 2 is joined to the surface. The protection part 2 can use what was produced from the ring made from stainless steel similarly to the protection part 2 used in Embodiment 1. FIG. All bonding can be performed with an epoxy adhesive.
[0187]
A dry gel layer to be the acoustic matching layer 3 was formed in the recess formed by the lower acoustic matching layer 8 and the protective part 2 in the same manner as in the first embodiment.
[0188]
In the present embodiment, in Step 1 similar to Step 1 in Embodiment 1, the density of the dried gel is adjusted by changing the concentration of ammonia serving as the gelation reaction catalyst, and the density of the acoustic matching layer is 0.2 ×. 10 ^Three kg / m ^Three , Sound speed 160m / s, acoustic impedance about 0.032 × 10 ⁶ kg / m ² A / s dry gel layer is formed. Since the sound velocity of the acoustic matching layer is 160 m / s, the height of the protection unit 2 is set to 80 μm so as to be 1/4 of the ultrasonic wavelength in the acoustic matching layer. It is preferable to perform the process which provides a hydroxyl group to the inner surface of the protection part 2 by plasma etching.
[0189]
FIG. 11A shows a transmission / reception waveform of the ultrasonic transceiver according to the present embodiment. In the graph of FIG. 11, the vertical axis represents signal amplitude and the horizontal axis represents time. The numerical value on the axis is an index notation, for example, “2.0E-04” is 2.0 × 10 ^-Four Means. The same applies to the other graphs.
[0190]
In the ultrasonic transmitter / receiver used for the measurement, the thickness of the porous ceramic to be the lower acoustic matching layer 8 is set to 1 mm, and the thickness of the protective portion 2 and the first acoustic matching layer (dry gel layer) 3 is set to 80 μm. did.
[0191]
For comparison, an ultrasonic transmitter / receiver using a conventional acoustic matching layer in which a glass balloon is hardened with epoxy instead of the two acoustic matching layers 3 and 8 in the ultrasonic transmitter / receiver of FIG. Transmitted and received waveforms were measured. The measurement results are shown in FIG.
[0192]
By using two acoustic matching layers, a sensitivity about 20 times higher than that of a conventional ultrasonic sensor could be obtained. In addition, it can be seen that, compared with the ultrasonic sensor in the first embodiment, the sensitivity is increased and the bandwidth is increased (short pulse). As described above, by making the acoustic matching layer into two layers, it is possible to provide an ultrasonic transceiver that is extremely suitable for transmitting and receiving pulses and burst waves.
[0193]
(Embodiment 6)
A sixth embodiment of the present invention will be described with reference to FIG.
[0194]
In this embodiment, a part of the lower acoustic matching layer 9 functions as a protective part, and the acoustic matching layer 9 and the protective part are integrated. In this example, the acoustic matching layer 9 is processed to form a recess on its main surface. The dry gel layer to be the upper acoustic matching layer 3 is formed in the recess of the lower acoustic matching layer 9.
[0195]
By employ | adopting such a structure, the process of joining a protection part to the acoustic matching layer 9 can be skipped. In addition, the problem that the height of the protective portion varies due to the adhesive layer can be solved, and an ultrasonic transceiver that operates with high sensitivity in a wide band can be manufactured with high yield.
[0196]
In the present embodiment, the lower acoustic matching layer 9 is formed of a porous body. For this reason, the coupling | bonding with the upper acoustic matching layer 3 is strong, and it can ensure high sensitivity and high stability. In order to further improve the adhesion, it is preferable to perform plasma treatment or acid treatment for imparting a hydroxyl group to the contact surface between the upper acoustic matching layer 3 and the lower acoustic matching layer 9 in advance.
[0197]
In this embodiment and Embodiment 5 described above, the acoustic matching layer has a two-layer structure, but the ultrasonic transducer of the present invention is not limited to such a configuration, and has a multilayer structure of three or more layers. You may have. By multilayering the acoustic matching layer, the sensitivity can be further increased and the band can be widened. However, in order to increase the sensitivity by increasing the number of layers, it is necessary to form an acoustic matching layer that is the outermost layer from a material having an extremely low acoustic impedance. Therefore, it is practical to adopt a two-layer structure.
[0198]
(Embodiment 7)
The seventh embodiment of the present invention will be described with reference to FIGS.
[0199]
As shown in FIG. 13, the ultrasonic transmitter / receiver according to the present embodiment has a back load material 10 bonded to the back side of the piezoelectric body 4, and the protection unit 2 is formed above the back load material 10. In other respects, the configuration is the same as that of the third embodiment.
[0200]
The back load material 10 has a function of attenuating ultrasonic waves radiated from the piezoelectric body 4 to the back side, and any material can be used as long as it can exhibit such a function. Also good.
[0201]
The protection part 2 is made of a cylindrical metal and is bonded to the main surface of the back load material 10. The thickness of the protection unit 2 is equal to the total thickness of the piezoelectric body 4, the lower acoustic matching layer 8, and the upper acoustic matching layer 3. In the present embodiment, since the thickness of the piezoelectric body 4 is 3 mm, the thickness of the acoustic matching layer 8 is 1 mm, and the thickness of the acoustic matching layer 3 is 0.08 mm, the thickness of the protective portion 2 is 4 0.08 mm.
[0202]
The back load material 10 of this embodiment is formed from ferrite rubber. Ferrite rubber is a material in which iron powder is dispersed in rubber, and has a high sound wave attenuation rate. By bonding such a back load material 10 to the back surface of the piezoelectric body 4, it is possible to attenuate ultrasonic waves emitted from the back surface side of the piezoelectric body 4 and transmit / receive broadband (short pulse width) ultrasonic waves. It becomes possible.
[0203]
FIG. 14 shows a transmission / reception waveform measured for the ultrasonic transceiver having the configuration of FIG. Although the transmission / reception sensitivity of this embodiment is lower than that of the ultrasonic transmitter / receiver of the third embodiment, an operation in a wider band is realized, and an ultrasonic transmitter / receiver suitable for transmitting / receiving short pulses is configured can do.
[0204]
Instead of the back load material 10 shown in FIG. 13, the back load material 11 shown in FIG. 15 may be used. A portion of the back load material 11 shown in FIG. 15 functions as a protection portion, and the protection portion and the back load material are integrated. The back load material 11 has a configuration in which a recess is formed in a region excluding the peripheral portion of the main surface, and the piezoelectric body 4 is inserted into the recess and is bonded to the inner surface of the recess of the back load material 11. The depth of the concave portion provided in the back load material 11 is set to be larger than the height of the piezoelectric body 4, and if a dry gel layer serving as an acoustic matching layer is formed on the upper surface of the piezoelectric body 4 after insertion, FIG. 15 configurations are obtained. By adopting the back load material 11, the same broadband as the ultrasonic transmitter / receiver of FIG. 13 is achieved.
[0205]
(Embodiment 8)
With reference to FIG. 16, an embodiment of a method of manufacturing the ultrasonic transceiver shown in FIG. 12 will be described.
[0206]
First, as shown in FIG. 16A, the piezoelectric body 4 and the lower acoustic matching layer 9 are joined to the structural support 6. An adhesive can be used for joining. As described above, the piezoelectric body 4 is made of piezoelectric ceramics, and the structural support 6 is made of stainless steel. The lower acoustic matching layer 9 is made of porous ceramics having a recess on the upper surface. The concave portion is formed by processing the upper surface of a flat porous ceramic with a lathe or the like.
[0207]
Next, as shown in FIG. 16 (b), a dry gel to be an upper acoustic matching layer is formed in the concave portion of the acoustic matching layer 9 adhered to the structural support 6. The formation of the dried gel can be performed by the method described in the first embodiment.
[0208]
In order to sufficiently infiltrate the gel raw material into the acoustic matching layer 9 formed of porous ceramics, it is preferable to place the gel raw material in a vacuum or a reduced pressure atmosphere after pouring the gel raw material. In this way, in the present embodiment, the gel raw material is allowed to penetrate not only the portion functioning as the protective portion of the acoustic matching layer 9 but also the entire interior of the acoustic matching layer 9. By doing so, the dried gel can be firmly bonded to the lower acoustic matching layer 9 and the characteristics of the acoustic matching layer 9 can be made uniform (FIG. 16C). Hereinafter, this point will be described with reference to FIG.
[0209]
FIG. 17A shows a state where the gel raw material has sufficiently penetrated into the lower acoustic matching layer 9. Therefore, the lower acoustic matching layer 9 functions acoustically as a single layer, and the acoustic impedance is stepped from a relatively high value of the acoustic matching layer 9 to a relatively low value of the upper acoustic matching layer. It decreases to the shape.
[0210]
On the other hand, when the penetration of the gel raw material is insufficient, an acoustic matching layer having a substantially three-layer structure is formed as shown in FIG. In this case, the acoustic impedance of the lowermost layer (first layer) where the penetration of the gel raw material is insufficient is smaller than the set value, so that the acoustic impedance of the middle layer (second layer) is the largest. When the distribution of the acoustic impedance is as shown in FIG. 17B, the acoustic impedance is not reduced stepwise from the piezoelectric body (not shown) arranged in the lower part of the figure toward the gas as the ultrasonic propagation medium. In addition, since the characteristics of the ultrasonic transducer are deteriorated, it is preferable to sufficiently permeate the gel raw material.
[0211]
In the above embodiment, as shown in FIG. 16, after the step of fixing the lower acoustic matching layer 9 to the structural support 6, the step of forming the first acoustic matching layer 3 is performed. The order may be reversed. With reference to FIGS. 18A to 18D, another manufacturing method will be described.
[0212]
First, as shown in FIG. 18A, an acoustic matching layer 9 having a portion that functions as a protection portion is prepared. Next, as shown in FIG. 18 (b), the gel raw material is dropped into the concave portion of the acoustic matching layer 9, and the gel raw material is ground so that the height of the gel raw material in the concave portion matches the height of the protective portion. It penetrates the entire acoustic matching layer 9. After the gel material is cured and hydrophobized, the gel raw material is dried by a supercritical drying method, and the acoustic matching layer 3 made of the dried gel is formed on the lower acoustic matching layer 9 as shown in FIG. .
[0213]
Thereafter, as shown in FIG. 18D, the acoustic matching layer is bonded to the structure support 6 in a state where the piezoelectric body 4 is fixed. Note that the acoustic matching layer in FIG. 18C may be directly bonded to the piezoelectric body 4 without using the structural support 6.
[0214]
In addition, it is preferable to select an optimal pressurizing condition so that the dried gel is not destroyed by pressurization during bonding. Since the strength of the dried gel with respect to the stress in the compression direction is relatively high, the production yield hardly decreases in the bonding step.
[0215]
The materials of the acoustic matching layer 3 and the acoustic matching layer 9 are preferably selected so that the elastic modulus of the upper acoustic matching layer 3 and the elastic modulus of the lower acoustic matching layer 9 are close to each other. When both elastic moduli are close to each other, a uniform pressure can be applied to the entire bonding surface, and it becomes easy to manufacture a highly sensitive ultrasonic transceiver with a high yield.
[0216]
In the method shown in FIGS. 18A to 18D, since it is not necessary to handle the piezoelectric body and the structural support in the process of forming the dry gel on the acoustic matching layer 9, the equipment such as the drying apparatus is small. This makes it possible to manufacture an ultrasonic transducer at a low cost.
[0217]
In the dry gel formation process, there is a possibility that a chemical load may be applied to organic substances such as an adhesive layer. However, the adhesion part is not deteriorated by performing the adhesion process after the formation of the dry gel.
[0218]
(Embodiment 9)
Another embodiment of the present invention will be described with reference to FIG.
[0219]
A characteristic point of the present embodiment is that the protective portion 2 is formed not only in the outer peripheral portion of the region where the acoustic matching layer 3 is formed, but also in the region.
[0220]
When forming a dry gel layer from a gel raw material through a wet gel, irregularities may be formed on the upper surface of the dry gel layer. Further, when the wet gel is dried by a normal drying method without using supercritical drying, the shrinkage of the dry gel occurs, so that a recess as shown in FIG. 4 is easily formed in the dry gel.
[0221]
When the ultrasonic transmission / reception surface is wide, the above irregularities tend to be large, and even if the thickness of the acoustic matching layer is set to the optimum value, the thickness of the acoustic matching layer is actually increased from the optimum value depending on the location. It will shift.
[0222]
Since the speed of sound in the dry gel layer is extremely slow, it is necessary to form a very thin layer to function properly as an acoustic matching layer, and the allowable error range of the thickness is small.
[0223]
If a protective portion having a layout as shown in FIG. 19A or FIG. 19B is formed, an error in the thickness of the acoustic matching layer can be suppressed to within about ± 5% from the target value.
[0224]
As shown in FIG. 19 (a) or FIG. 19 (b), when the protective part 2 is also provided inside the ultrasonic radiation surface of the ultrasonic transceiver, the variation in the thickness of the acoustic matching layer 3 is minimized. However, the protection unit 2 provided on the inner side of the ultrasonic radiation surface can be an obstacle to transmission / reception of ultrasonic waves. In order to prevent acoustic characteristics from being deteriorated by the protection unit 2, it is preferable that the size of the protection unit 2 be as small as possible in a range that serves as the protection unit 2.
[0225]
In the configuration example of FIG. 19A, the protective portions 2 having a circular cross section are randomly arranged. However, the cross sectional shape of the protective portion 2 is not limited to a circular shape, and may be a rectangle or a polygon. Further, the arrangement is not limited to a random arrangement.
[0226]
In the configuration example of FIG. 19B, the concentric protection part 2 is provided. In this configuration example, the protection unit 2 is also present inside the ultrasonic radiation surface of the ultrasonic transceiver, but the characteristic deterioration of the ultrasonic transceiver is prevented. The configuration of FIG. 19B is effective when transmitting an ultrasonic wave to a position separated from the ultrasonic radiation surface by a short distance L on the central axis of the ultrasonic handset.
[0227]
When the distance between the protection unit 2 and the center of the ultrasonic radiation surface is r, the distance r preferably satisfies the following formula (6).
[0228]
[Expression 1]

[0229]
Here, λ is the wavelength in the gas through which the ultrasonic wave propagates, and L is the distance from the ultrasonic radiation surface of the ultrasonic transceiver. For example, when the frequency is 500 kHz, the ultrasonic propagation medium is air (sonic velocity 340 m / s), and the measurement distance L is 10 mm, the radius r from the center is 2.6 to 3.7 mm, 4.6 from Equation (6). It turns out that it is preferable to provide the protection part 2 in the position of -5.4 mm, 6.1-6.7 mm .... Providing the protection unit 2 at such a position is effective in preventing disturbance of the sound field due to sound interference and preventing deterioration in sensitivity of ultrasonic waves at a short distance.
[0230]
When each point on the sound wave emitting surface of the ultrasonic transceiver is regarded as a point sound source, a synthesized ultrasonic wave from each point sound source becomes an ultrasonic wave to be transmitted. When the distance from the ultrasonic radiation surface is short, ultrasonic waves having different phases cancel each other, and there is a position where high-power ultrasonic waves cannot be transmitted. In order to radiate only ultrasonic waves having the same phase from the ultrasonic radiation surface, it is effective to provide the protection unit 2 in a region where ultrasonic waves having different phases are radiated. When the protection unit 2 is provided in such a region, it is possible to suppress the emission of ultrasonic waves having different phases, so that the disturbance of the sound field at a short distance can be prevented and high-power ultrasonic transmission can be performed.
[0231]
(Embodiment 10)
FIG. 20 is a sectional view showing a tenth embodiment of the ultrasonic transducer according to the present invention. The ultrasonic transducer 21 of the present embodiment includes a piezoelectric body 22, electrodes 23a and 23b provided on both surfaces of the piezoelectric body 22, and a protective matching layer (first layer) provided on the piezoelectric body 22 via the electrode 23a. 1 acoustic matching portion) 24 and an acoustic matching layer (second acoustic matching portion) 25 provided on the piezoelectric body 22 via the electrode 23a.
[0232]
FIG. 21 is a top view of the ultrasonic transducer 21 shown in FIG. As can be seen from FIG. 21, the ultrasonic transducer of this embodiment has a structure in which protective matching layers 24 and acoustic matching layers 25 having different thicknesses (heights) are alternately arranged in a concentric manner. Yes.
[0233]
The piezoelectric body 22 in the present embodiment is formed of a material having piezoelectricity and is polarized in the thickness direction. When a voltage is applied to the electrodes 23 a and 23 b provided on the upper and lower surfaces of the piezoelectric body 22, an ultrasonic wave is generated in the piezoelectric body 22 based on the voltage signal, and the ultrasonic wave is transmitted through the protective matching layer 24 and the acoustic matching layer 25. Radiated to a sound wave propagation medium (gas or the like) 26. Further, the ultrasonic wave propagated through the ultrasonic propagation medium 26 propagates to the piezoelectric body 22 through the protective matching layer 24 and the acoustic matching layer 25. The piezoelectric body 22 is deformed by the incident ultrasonic wave, and a voltage signal is generated between the electrode 23a and the electrode 23b.
[0234]
The material of the piezoelectric body 22 is arbitrary, and those formed from various known materials can be used. A known electrostrictive body may be used instead of the piezoelectric body 22. The electrodes 23a and 23b are preferably made of metal, but may be made of a conductive material other than metal.
[0235]
The protective matching layer 24 and the acoustic matching layer 25 efficiently propagate the ultrasonic vibration generated in the piezoelectric body 22 to the propagation medium 26, and efficiently transmit the ultrasonic wave propagated through the ultrasonic propagation medium 26 to the piezoelectric body 22. Has the ability to communicate.
[0236]
The acoustic matching layer 25 of the present embodiment is preferably formed from a dry gel. The dry gel is a porous body formed by a sol-gel reaction, and is a material capable of extremely reducing acoustic impedance defined by the product of density ρ and sound velocity C (ρ × C). For this reason, by using the acoustic matching layer 25 formed from a dry gel, the transmission and reception efficiency of ultrasonic waves with respect to a gas such as air can be extremely increased.
[0237]
A dry gel is obtained by forming a wet gel and then drying the wet gel. As the wet gel, first, a gel raw material solution is prepared, and a wet gel can be prepared by reaction of the gel raw material solution. The wet gel has a solid skeleton that is solidified by the reaction of the gel raw material solution, and the solid skeleton contains a solvent.
[0238]
The dry gel obtained by drying the wet gel is a porous body and has continuous pores in the gaps of the solid skeleton part of about several nm to several μm. The average pore size is as small as 1 nm to several μm.
[0239]
When the density of the dried gel is reduced by adjusting the production conditions, the sound velocity in the solid portion of the dried gel becomes extremely small, and the sound velocity in the gas portion in the pores becomes extremely small. Therefore, the sound speed of the dry gel shows a low value of 500 m / sec or less in a low density state, and shows a very low acoustic impedance. In particular, a dry gel having a solid skeleton portion and a pore size as small as several nanometers exhibits an extremely low sound velocity. In addition, since the pressure loss of gas is large in the nanometer-sized pores, when the acoustic matching layer is formed from the dried gel, sound waves can be emitted with a high sound pressure.
[0240]
According to the manufacturing method described later, the acoustic impedance of the dried gel can be arbitrarily controlled within a wide range by adjusting the manufacturing process conditions even when the same raw material is used. Also, by changing the manufacturing process conditions, it is possible to produce an acoustic matching layer in which only the sound speed is changed while the density is substantially the same.
[0241]
Dried gels have such advantageous characteristics but have low mechanical strength. For this reason, it was difficult to increase the manufacturing yield, and the reliability during use was also low. As described above with respect to the first to ninth embodiments, the production yield and reliability are improved by providing the member that protects the dried gel having low mechanical strength.
[0242]
The protection unit in the first to ninth embodiments is extremely effective for improving the manufacturing yield of the ultrasonic transducer or the reliability in use, and can further control the thickness of the acoustic matching layer with high accuracy. This is effective for stabilizing the performance of the ultrasonic transducer. However, as described above, when a protective portion is provided on the surface (main surface) where the piezoelectric body emits or receives ultrasonic waves, the protective portion may be an acoustic obstacle. This is because, in the first to ninth embodiments, the thickness of the protective portion formed from a material different from the material of the acoustic matching layer is set to be substantially equal to the thickness of the acoustic matching layer. This is because the speed of sound is different between the protective portion and the acoustic matching layer, and the protective portion having the same thickness as the acoustic matching layer does not serve as the acoustic matching layer. For this reason, since the protection part of Embodiments 1-9 may become a factor which obstructs transmission / reception of an ultrasonic wave, it is preferable to arrange | position outside the main surface of a piezoelectric material.
[0243]
However, there are cases where it is necessary to provide a protective part on the upper part of the piezoelectric body depending on ensuring reliability against more severe environmental conditions and limiting the outer diameter of the ultrasonic transducer.
[0244]
In the present embodiment, a protection portion (formed from a material having a relatively high density and higher mechanical strength than the acoustic matching layer 25) is provided on the main surface of the piezoelectric body. A configuration that does not impair the performance as an ultrasonic transducer while having it is adopted.
[0245]
In the present embodiment, the thickness of the protective portion provided on the main surface of the piezoelectric body 22 is set to about ¼ of the wavelength of the ultrasonic wave that is transmitted and received. Thereby, the protective part having a relatively high mechanical strength also functions as an acoustic matching layer. For this reason, in this specification, such a protection part may be referred to as a “protective matching layer”. By adopting such a configuration, the protection unit that protects the acoustic matching layer also plays a role as the acoustic matching layer, so that a highly sensitive ultrasonic transducer can be realized.
[0246]
The thickness that best functions as an acoustic matching layer is 1/4 of the wavelength of the ultrasonic wave. On the other hand, the speed of sound in the protective matching layer 24 and the speed of sound in the acoustic matching layer 25 are different. Therefore, the thickness L3 of the protective matching layer 24 and the thickness L1 of the acoustic matching layer 25 have different sizes as shown in FIG. 20 (L3> L1).
[0247]
If the thickness of the protective matching layer 24 and the acoustic matching layer 25 are both set to about 1/4 of the sound velocity, the thickness of the protective matching layer 24 is different from the thickness of the acoustic matching layer 25. In some cases, the ultrasonic wave emitted from the upper surface interferes with the ultrasonic wave emitted from the upper surface of the protective matching layer 24. In order to realize a highly sensitive ultrasonic transducer, the phase relationship between the ultrasonic waves radiated from each is extremely important.
[0248]
22A shows an ultrasonic waveform on the upper surface of the protective matching layer 24, and FIG. 22B shows an ultrasonic waveform on the same level as the upper surface of the protective matching layer 24 above the acoustic matching layer 25. Is shown. Note that the symbol “ta” in FIG. 22B indicates the time during which the ultrasonic wave propagates through the ultrasonic wave propagation medium 26. One scale on the horizontal axis in each graph is about 3 μs when the ultrasonic frequency is 500 kHz.
[0249]
The ultrasonic wave emitted from the upper surface of the acoustic matching layer 25 reaches the same level as the upper surface of the protective matching layer 24 through the ultrasonic propagation medium 26 such as a gas. For this reason, the phase relationship of the waveform of the ultrasonic wave at the same level as the upper surface of the protective matching layer 24 changes above the acoustic matching layer 25 depending on the speed of sound in the propagation medium 26 and the size L2 of the propagation medium 26.
[0250]
Note that the signal waveforms in FIGS. 22A and 22B are obtained on the assumption that the wavelengths and amplitudes of the ultrasonic waves radiated from the protective matching layer 24 and the acoustic matching layer 25 are equal.
[0251]
When the thickness L3 of the protective matching layer 24 and the thickness L1 of the acoustic matching layer 25 are each 1/4 of the ultrasonic wavelength in each layer, ultrasonic waves propagate between the lower surface and the upper surface of the protective matching layer 24. The time required for this is equal to the time required for the ultrasonic wave to propagate between the lower surface and the upper surface of the acoustic matching layer 25. Therefore, the phase of the ultrasonic wave that the ultrasonic wave radiated from the upper surface of the acoustic matching layer 25 reaches the same level as the upper surface of the protective matching layer 24 propagates through the protective matching layer 24 and reaches the upper surface of the protective matching layer 24. It is delayed compared to the phase of the reached ultrasonic wave. This phase delay corresponds to the time required for the ultrasonic wave emitted from the upper surface of the acoustic matching layer 25 to propagate through the propagation medium 26 by the distance L2.
[0252]
The frequency of ultrasonic waves to be sent and received is f [seconds] ^-1 ], The time required for the ultrasonic wave to travel by a distance equal to one wavelength of the ultrasonic wave is 1 / f [second]. The time t3 required for the ultrasonic wave to pass through the protective matching layer 24 of this embodiment is ¼ f [second]. On the other hand, the time t2 required for the ultrasonic wave to pass through the acoustic matching layer 25 of the present embodiment is also 1/4 f [second]. Here, if the time required for the ultrasonic wave to propagate through the propagation medium 26 by the distance L2 is t2 (= ta), the ultrasonic wave radiated from the upper surface of the protective matching layer 24 depends on the time t2. Interference occurs between the sound wave and the ultrasonic wave emitted from the upper surface of the acoustic matching layer 25. This interference changes the ultrasonic waveform and sensitivity.
[0253]
FIG. 22C shows an ultrasonic waveform observed when the time t2 is 1 / 2f [second], and FIG. 22D shows a case where the time t2 is 1 / f [second]. Shows the observed ultrasonic waveform. As can be seen from FIGS. 22C and 22D, the sensitivity of the observed ultrasonic wave varies greatly depending on the value of time t2. When the time t2 is equal to 1/2 f [second], the phase shift becomes the half wavelength of the ultrasonic wave, and the sensitivity of the observed ultrasonic wave is low. On the other hand, when the time t2 is 1 / f [second], the phase shift is an integral multiple of the wavelength of the ultrasonic transducer, so that the sensitivity of the observed ultrasonic wave is high. When the time t2 is in the range of 1 / 2f to 1 / f [second], the transmission / reception sensitivity of the ultrasonic wave increases as t2 approaches 1 / f [second] from 1 / 2f [second].
[0254]
When the ultrasonic wave radiated from the acoustic matching layer 25 propagates through the propagation medium 26 and reaches the same level as the upper surface of the protective matching layer 24, the phase of the ultrasonic wave propagates through the protective matching layer 24. By adjusting the thicknesses of the acoustic matching layer 25 and the protective matching layer 24 so as to substantially coincide with each other, a highly sensitive ultrasonic transducer can be provided. In the present specification, when “the phases are substantially matched”, it means that the difference in phase of the ultrasonic wave is ¼ or less of the ultrasonic wavelength, and the smaller the phase difference is, the better.
[0255]
FIG. 23 is a cross-sectional view schematically showing the phase of an ultrasonic wave when the time t2 is 1 / f [second]. In this figure, the phase of the ultrasonic wave on the upper surface of the protective matching layer 24 and the phase of the ultrasonic wave above the acoustic matching layer 25 and at the same level as the upper surface of the protective matching layer are the same. When such phase matching occurs, the ultrasonic wave transmission / reception sensitivity is maximized. Even when such complete phase matching does not occur, if the phase shift is set to be small, the transmission / reception sensitivity of the ultrasonic wave is sufficiently improved as compared with the prior art. The phase shift is preferably adjusted to 1/4 or less of the ultrasonic wavelength in the ultrasonic propagation medium, and more preferably adjusted to 1/8 or less.
[0256]
By simply controlling the thickness L1 of the acoustic matching layer 25 and the thickness L3 of the protective matching layer 24 to about 1/4 of the ultrasonic wavelength in the acoustic matching layer 25 and the protective matching layer 24, respectively, the magnitude of L2 is Since it is uniquely determined as (L3-L1), t2 cannot be set arbitrarily. Therefore, in order to set the time t2 to a desired magnitude, not only the thickness of the acoustic matching layer 25 and the protective matching layer 24 but also the sound velocity in the acoustic matching layer 25 and the protective matching layer 24 is appropriately controlled. There is a need. In a preferred embodiment of the present invention, the acoustic matching layer 25 is formed from a dry gel whose sound speed can be easily controlled.
[0257]
Next, an embodiment of a method for manufacturing the ultrasonic transducer 21 of the present embodiment will be described with reference to FIGS. In the present embodiment, the ultrasonic propagation medium 26 is air (density: 1.18 kg / m ^Three , Sound speed: about 340 m / s, acoustic impedance about 4.0 × 10 ² kg / m ² / S).
[0258]
First, as shown in FIG. 24A, a piezoelectric body 22 is prepared according to the wavelength of ultrasonic waves to be transmitted and received. The piezoelectric body 22 at this stage is not provided with the protective matching layer 24 shown in FIG. The piezoelectric body 22 is preferably made of a highly piezoelectric material such as piezoelectric ceramics or a piezoelectric single crystal. As the piezoelectric ceramic, lead zirconate titanate, barium titanate, lead titanate, lead niobate, or the like can be used. As the piezoelectric single crystal, lead zirconate titanate single crystal, lithium niobate, quartz, or the like can be used.
[0259]
In the present embodiment, lead zirconate titanate ceramics are used as the piezoelectric body 22, and the wavelength of ultrasonic waves to be transmitted and received is set to 500 kHz. In order to allow the piezoelectric body 22 to efficiently transmit and receive such ultrasonic waves, the resonance frequency of the piezoelectric body 22 is designed to be 500 kHz. For this reason, in this embodiment, the piezoelectric body 22 formed from a piezoelectric ceramic having a cylindrical shape with a diameter of 12 mm and a thickness of about 3.8 mm is used. Electrodes 23a and 23b are formed on both surfaces of the piezoelectric body 22 by baking of silver, and polarization treatment is performed in this direction.
[0260]
Next, three ring-shaped members functioning as the protective matching layer 24 are prepared and bonded to the main surface of the piezoelectric body 22 as shown in FIG. At this time, the centers of the ring-shaped members are aligned with the centers of the piezoelectric bodies 22 as shown in FIG. The three ring-shaped members functioning as the protective matching layer 24 are respectively a first ring-shaped member having an outer diameter of 12 mm, an inner diameter of 11 mm, and a thickness of 1.0 mm, an outer diameter of 8 mm, an inner diameter of 7 mm, and a thickness of 1.0 mm. 2 ring-shaped members and a third ring-shaped member having an outer diameter of 4 mm, an inner diameter of 3 mm, and a thickness of 1.0 mm.
[0261]
The protective matching layer 24 in this embodiment is not only required to have a high mechanical strength and a function capable of protecting the acoustic matching layer, but also to have a relatively low acoustic impedance in order to fulfill the function of the acoustic matching layer. It is done. In this embodiment, a porous ceramic is used as such a material. This porous ceramic has an apparent density of 0.64 × 10 6. ^Three kg / m ^Three , Sound speed is 2000m / s, acoustic impedance is about 1.28 × 10 ⁶ kg / m ^Three It is. As the ceramic, a barium titanate-based material is used. The “apparent density” is a density including a space part included in the porous body. About 80% of the volume of the porous ceramic is a space portion, and the substantial portion of the ceramic is about 20% by volume.
[0262]
As described above, since the sound velocity of the protective matching layer 24 is about 2000 m / s, the quarter thickness of the wavelength at 500 kHz corresponds to 1.0 mm. For this reason, in this embodiment, the thickness of the ring-shaped member that functions as the protective matching layer 24 is set to 1.0 mm.
[0263]
The porous ceramic used in the present embodiment can be produced as follows.
[0264]
First, resin-made fine balls and ceramic powder are mixed and pressed. Thereafter, the ceramic is sintered. In this sintering process, the resin balls are heated, burned and removed. When heating is performed rapidly during sintering, the resin balls may expand or rapidly gasify, and the ceramic structure may be destroyed. For this reason, the sintering is preferably performed by gentle heating.
[0265]
In the present embodiment, the protective matching layer 24 formed from such porous ceramics and the piezoelectric body 22 are joined by bonding with an adhesive. For example, an epoxy resin is used as an adhesive, and when left in a thermostatic bath at 150 ° C. for about 2 hours while applying a pressure of about 0.1 MPa, the adhesive is cured, and the protective matching layer 24 and the piezoelectric body 22 Join.
[0266]
Next, an acoustic matching layer 25 is provided on the composite body formed of the piezoelectric body 22 / protective matching layer 24 as shown in FIG. In the present embodiment, the acoustic matching layer 25 is formed from a dry gel.
[0267]
In the present embodiment, first, the acoustic matching layer 25 having the thickness shown in FIG. 24B is formed, and then the acoustic matching layer 25 is thinned as shown in FIG. At this time, the thickness L3 of the protective matching layer 24 and the thickness L1 of the acoustic matching layer are set so that the distance L2 (= L3-L1) shown in FIG. 20 is equal to one wavelength of the ultrasonic wave in the air. Specifically, since the frequency of ultrasonic waves to be transmitted and received is 500 kH, one wavelength of the ultrasonic waves in the air is 0.62 mm. On the other hand, since the thickness L3 of the protective matching layer 24 is 1.0 mm, the thickness L1 of the acoustic matching layer 25 is 0.32 mm (= 1.0 mm−0.62 mm). In order for the acoustic matching layer 25 to function properly as the acoustic matching layer, the thickness L1 (= 0.32 mm) is most preferably ¼ of the wavelength of the ultrasonic wave propagating through the acoustic matching layer 25. desirable. Therefore, it is necessary to have a material characteristic having a sound speed such that 0.32 mm is 1/4 of the wavelength of the ultrasonic wave transmitted and received. According to the calculation, the acoustic matching layer 25 having a thickness of 0.32 mm may be formed from a dry gel having a sound speed of 640 m / s.
[0268]
The thickness of the protective matching layer 24 is preferably ¼ of the ultrasonic wavelength in the protective matching layer 24, but is not limited to that size. It may be in the range of 1/8 to 1/3 of the ultrasonic wavelength, and more preferably in the range of 1/6 to 1/4 of the ultrasonic wavelength. When there is a distribution in the wavelength of the ultrasonic wave, it is preferable to determine the thickness based on the peak wavelength. In this specification, when there is a distribution of wavelengths, “1/4 of wavelength” means “1/4 of peak wavelength”.
[0269]
When the acoustic matching layer 25 is a single layer, the thickness of the acoustic matching layer 25 is also preferably ¼ of the ultrasonic wavelength in the acoustic matching layer 25, but is not limited to that size. It may be in the range of 1/8 to 1/3 of the ultrasonic wavelength, and more preferably in the range of 1/6 to 1/4 of the ultrasonic wavelength. When the acoustic matching layer has a multilayer structure, it is preferable that each constituent layer has the above thickness. An ultrasonic transducer in which the acoustic matching layer has a multilayer structure will be described later as a second embodiment.
[0270]
As a material of the dry gel constituting the acoustic matching layer 25, various materials such as an inorganic material and an organic polymer material can be used. As the solid skeleton portion of the inorganic material, silicon oxide (silica), aluminum oxide (alumina), titanium oxide, or the like can be used. Further, as the solid skeleton portion of the organic material, a general thermosetting resin or thermoplastic resin can be used. For example, polyurethane, polyurea, phenol resin, polyacrylamide, polymethyl methacrylate, or the like can be used.
[0271]
In the present embodiment, as a material of the acoustic matching layer 25, silicon oxide (silica) is used as a solid skeleton portion from the viewpoint of cost, environmental stability, ease of manufacturing, stable temperature characteristics of the ultrasonic transducer, and the like. Adopt dry gel.
[0272]
The sound speed of 640 m / s is a relatively high value as the sound speed of the dry gel. For this reason, in this embodiment, when a dry gel layer is formed as the acoustic matching layer 25, a conventional manufacturing method in which a drying step is performed subsequent to a gelation step (hereinafter referred to as "first gelation step"). Instead, a method of performing the second gelation step after the first gelation step is adopted.
[0273]
When only the first gelation step is performed without performing the second gelation step, it is difficult to obtain a dry gel showing a relatively high sound speed. In addition, since the density of the dried gel increases in proportion to the sound speed, “high sound speed” means “high density”. If the concentration of the gel raw material in the gel raw material liquid is increased for the purpose of increasing the sound speed of the gel, the gelation reaction does not proceed uniformly, and a wet gel having a random sound speed distribution is formed. A dry gel obtained by drying the wet gel also has a random density distribution. For this reason, when the concentration of the gel raw material in the gel raw material liquid is increased, it becomes extremely difficult to make the sound speed uniform.
[0274]
In this embodiment, in order to avoid gel non-uniformity, the speed of sound of the dried gel formed in the first gelation step is adjusted to about 200 m / s or less, and the density is further increased by the second gelation step. Increase the speed of sound. In the second gelation step, the wet gel obtained in the first gelation step is again immersed in the gel raw material liquid (second gelation raw material liquid). In the second gelation step, the concentration of ammonia serving as a catalyst in the second gelation raw material liquid is adjusted to be low. For this reason, gelation does not occur outside the wet gel obtained in the first gelation step. However, inside the wet gel obtained in the first gelation step, the second gelation raw material liquid grows so as to adhere to the skeleton formed in the first gelation step. For this reason, this reaction proceeds even under conditions where the gel raw material liquid itself does not gel. In this way, the sound speed and density of the gel can be changed.
[0275]
Specifically, the acoustic matching layer 25 made of dry gel is formed by performing the following steps.
[0276]
Step 1: Preparation of first gelled gel raw material liquid
Tetraethoxysilane / ethanol / water / hydrogen chloride are mixed at a molar ratio of 1/2/1 / 0.00078, and the hydrolysis of tetraethoxysilane is allowed to proceed for 3 hours in a thermostatic bath at 65 degrees. In addition, water / NH _Three Is prepared by adding a ratio of 2.5 / 0.0057 (molar ratio to tetraethoxysilane) and mixing them.
[0277]
Process 2: First gelation process
The gel raw material liquid (first gelation raw material liquid) adjusted as described above is dropped into the space formed by the piezoelectric body 22 and the protective matching layer 24. At this time, a Teflon sheet is wound around the outer periphery of the outermost protective matching layer 24 to form a frame so that the gel raw material liquid does not spill.
[0278]
The sample to which the gel raw material liquid has been dropped is left at 50 ° C. for about 1 day while keeping the level in a thermostatic bath. Thus, the gel raw material liquid supplied in the space formed by the piezoelectric body 22 and the protective matching layer 24 gels to form a wet gel.
[0279]
Step 3: Second gelation step (adjustment of sound speed and density)
When the acoustic matching layer obtained in the first gelation step is directly subjected to the drying step, the density is 2.0 × 10. ² kg / m ^Three The sound speed is about 200 m / s. In the present embodiment, the second gelation step is performed for the purpose of further increasing the sound speed and density.
[0280]
First, the wet gel obtained in the first gelation step is washed with ethanol to prepare a second gelation raw material liquid. As the second gelation raw material liquid, a mixture of tetraethoxysilane / ethanol / 0.1N aqueous ammonia and 60/35/5 in volume ratio is used.
[0281]
The composite composed of the piezoelectric material 22 / wet gel / protective matching layer 24 obtained in the first gelation step is immersed in the second gelation raw material liquid in a sealed container and left in a constant temperature bath at 70 ° C. for about 48 hours. To do. By this second gelation step, the gel skeleton obtained in the first gelation step grows, and the density and sound speed increase.
[0282]
Process 4: Hydrophobization process
The hydrophobizing step is not necessarily required, but it is preferable to perform the hydrophobizing step because the performance may deteriorate due to moisture absorption. In the hydrophobization step, after the second gelation step, the second gelation raw material liquid remaining in the wet gel is replaced and washed with ethanol, and then dimethyldimethoxysilane / ethanol / 10 wt% ammonia water is added. The hydrophobizing step is performed by immersing in a hydrophobizing solution obtained by mixing at a weight ratio of 45/45/10 at 40 ° C. for about 1 day.
[0283]
Process 5: Drying process
In order to obtain a dry gel from the wet gel obtained in the above steps, a drying step is performed. In this embodiment, a supercritical drying method is used as the drying method. As mentioned above, a dry gel is a very small nanometer-sized porous material. Depending on the thickness of the skeleton, the strength of the bond, and the size of the pores, solvent drying from the wet gel to the dry gel is possible. In this case, it may be destroyed by the surface tension of the solvent.
[0284]
For this reason, the supercritical drying method in which surface tension does not work can be used effectively. Specifically, after replacing the above hydrophobized liquid with ethanol, the composite of piezoelectric body 22 / wet gel / protective part polishing acoustic matching layer 25 obtained in the above steps is put in a pressure resistant container, and the wet gel is added. The ethanol inside is replaced with liquefied carbon dioxide.
[0285]
Furthermore, the pressure in the container is increased to 10 MPa by sending liquefied carbon dioxide into the container with a pump. Then, the inside of a container was made into the supercritical state by heating up to 50 degreeC. Next, drying is completed by slowly releasing the pressure while maintaining the temperature at 50 ° C.
[0286]
Process 6: Thickness adjustment process
The dried gel layer thus formed was ground on the acoustic matching layer 25 only by a lathe so that the thickness was 0.32 mm.
[0287]
The density of the dried gel forming the acoustic matching layer 25 thus obtained is about 0.6 × 10 ^Three kg / m ^Three The sound speed is about 640 m / s. Moreover, although the dry gel used as the acoustic matching layer 25 has permeated into a part of the protective matching layer 24, the sound velocity of the protective matching layer 24 is not affected.
[0288]
Before the step of forming the acoustic matching layer from the dried gel, it is preferable to treat the surface of the electrode 23b so that the adhesion between the electrode 23b and the acoustic matching layer 25 is improved. When the adhesion between the electrode 23b and the acoustic matching layer 25 is increased by the surface treatment, the reliability is further improved. As such a surface treatment, a plasma treatment or the like in which a hydroxyl group is imparted to an electrode on the surface of a piezoelectric body that is easily chemically bonded to a dry gel can be employed. Alternatively, it is also effective to impart an anchor effect by forming physical irregularities on the surface of the electrode 23b. Specifically, a chemical and / or physical etching process can be suitably employed.
[0289]
In this embodiment, after forming the gel used as the acoustic matching layer 25, it grinds with a lathe and adjusts the thickness of a dry gel. The adjustment of the thickness may be performed by adjusting the amount (height) of the first gelation raw material liquid dropped during the first gelation step. In this case, 33.9 μL of the gel raw material liquid is accurately weighed with a micropipette and dropped onto the piezoelectric body 22 so that the thickness of the acoustic matching layer to be formed is finally about 0.32 mm. . Since the protective matching layer 24 is a porous body having 80% voids, it is necessary to calculate the dripping amount by converting the volume absorbed by the porous body.
[0290]
The transmission / reception waveform of the ultrasonic transducer thus manufactured is shown in FIG. In FIG. 25, the ultrasonic transmission / reception waveform of the present embodiment is indicated by a solid line, and the ultrasonic transmission / reception waveform of an ultrasonic transmitter / receiver (comparative example) in which the protective portion and the acoustic matching layer 25 have the same thickness is indicated by a dotted line. Has been. As can be seen from FIG. 25, according to this embodiment, the amplitude of the signal increases.
High sensitivity can be achieved by using the structure of the present invention.
[0291]
In the present embodiment, in order to align the phase of the ultrasonic wave at the same level as the upper surface of the protective matching layer 24, the ultrasonic wave that has propagated through the acoustic matching layer 25 and the propagation medium 26 has propagated through the protective matching layer 24. Compared to sound waves, the phase is delayed by exactly the wavelength. When the protective matching layer 24 is formed from a material having a higher sound velocity, or when the thickness L3 of the protective matching layer 24 is set to be large, the phase delay due to the propagation medium 26 is set to two or more ultrasonic wavelengths. May be.
[0292]
(Embodiment 11)
Next, an eleventh embodiment of the ultrasonic transducer of the present invention will be described with reference to FIG. The main feature of this embodiment is that the acoustic matching layer has a laminated structure including a lower first acoustic matching layer 25a and an upper second acoustic matching layer 25b.
[0293]
Even when the acoustic matching layer 25 has a two-layer structure, the thickness of each acoustic matching layer 25a, 25b is preferably set to about 1/4 of the ultrasonic wavelength in each acoustic matching layer.
[0294]
Also in the present embodiment, as in the tenth embodiment, in order to align the phase of the ultrasonic wave at the same level as the upper surface of the protective matching layer 24, the ultrasonic wave propagated through the acoustic matching layer 25 and the propagation medium 26 is Compared to the ultrasonic wave propagating through the protective matching layer 24, a phase delay corresponding to substantially an integral multiple of the ultrasonic wavelength is generated.
[0295]
In the configuration shown in FIG. 26, when an ultrasonic wave propagates through the protective matching layer 24 and reaches the upper surface of the protective matching layer 24, the ultrasonic wave having the same phase as that of the ultrasonic wave has a first acoustic matching layer and a second acoustic matching layer. The boundary surface of 25b is reached. This is because the sound speed of the first acoustic matching layer 25 a is smaller than the sound speed of the protective matching layer 24. It takes an additional ¼ f [second] for the ultrasonic wave to reach the upper surface of the second acoustic matching layer 25b from the upper surface of the first acoustic matching layer 25a. Therefore, if the time until the propagation medium 26 propagates from the upper surface of the acoustic matching layer 25b to reach the same level as the upper surface of the protective matching layer 24 is set to 3 / 4f [seconds], the upper surface of the protective matching layer 24 is set. The phase is aligned at the level. When such a configuration is adopted, a shift of one wavelength occurs between the ultrasonic wave transmitted through the acoustic matching layers 25a and 25b and the ultrasonic wave transmitted through the protective matching layer 24. Since both ultrasonic waves interfere and strengthen each other, the amplitude of the ultrasonic waves increases.
[0296]
A method for manufacturing the acoustic matching layers 25a and 25b in the present embodiment will be described.
[0297]
First, the protective matching layer 24 is produced in the same manner as the manufacturing method of the acoustic matching layer 25 in the tenth embodiment. Porous ceramics is used as the material of the protective matching layer 24, and its thickness (L7) is set to 1.0 mm.
[0298]
In the present embodiment, the distance (L6) from the upper surface of the second acoustic matching layer 25b to the upper surface level of the protective matching layer 24 is set so that the time for propagating in the propagation medium 26 such as air is 3 / 4f [seconds]. Set to 0.51 mm. As a result, the total thickness (L4 + L5) of the first acoustic matching layer 25a and the second acoustic matching layer 25b is equal to 0.49 mm.
[0299]
In the present embodiment, when the sound speed of the second acoustic matching layer 25b is set to 200 m / s, the thickness (L5) of the second acoustic matching layer 25b is preferably set to 0.10 mm. When L5 = 0.10 mm, the thickness (L4) of the first acoustic matching layer 25a is 0.39 mm (= 0.49 mm−0.10 mm). In order for the thickness of the first acoustic matching layer 25a to correspond to a quarter wavelength of the ultrasonic wave in the first acoustic matching layer 25a, the sound speed of the first acoustic matching layer 25a needs to be 780 m / s. .
[0300]
Next, a manufacturing method of the two acoustic matching layers 25a and 25b will be described. A characteristic point of this method is that the second gelation step performed in the tenth embodiment is performed twice. That is, in this embodiment, after performing the 2nd gelation process (2-1 gelation process) which does not gel on the outside of the wet gel formed at the 1st gelation process, it is gel also on the outside of the wet gel. A second gelation step (second-2 gelation step) in which crystallization occurs is performed.
[0301]
In the present embodiment, first, the first acoustic matching layer 25a is formed by performing the same processes as the processes 1 to 6 performed in the tenth embodiment. However, the second gelation step at this time is a 2-1 gelation step.
[0302]
In view of an increase in acoustic impedance in the 2-2 gelling step performed thereafter, in the 2-1 gelling step, the density of the first acoustic matching layer 25a is about 0.5 × 10. ^Three kg / m ^Three The processing time is adjusted so that the sound speed is about 500 m / s. In the present embodiment, the processing time is set to about 36 hours, which is shorter than the processing time of the second gelation step in the tenth embodiment.
[0303]
Next, by performing the 2-2 gelling step, the acoustic impedance of the first acoustic matching layer 25a is increased, and the second acoustic matching layer 25b is formed on the first acoustic matching layer 25a. Specifically, the 2-2 gelation step was performed as follows.
[0304]
2-2 Gelation step:
First, a liquid in which tetraethoxysilane / ethanol / 0.05N aqueous ammonia is mixed at a molar ratio of 1/4/3 is prepared as the 2-2 gelling raw material liquid. This 2-2 gelling raw material liquid is filled in the space formed by the first acoustic matching layer 25a and the protective matching layer 24. Next, the gelation is completed by leaving it at room temperature for about 24 hours. Thus, the acoustic impedance of the first acoustic matching layer 25a is adjusted, and a wet gel that becomes the second acoustic matching layer 25b is formed.
[0305]
Thereafter, the acoustic matching layers 25a and 25b are completed by performing the hydrophobizing step, the drying step, and the thickness adjusting step in the same manner as in the tenth embodiment. The acoustic matching layers 25a and 25b in the present embodiment are characterized as follows.
[0306]
First acoustic matching layer 25a
Density: 0.7 × 10 ^Three kg / m ^Three , Sound speed: 780m / s
Acoustic impedance: 5.46 × 10 ^Five kg / m ² / S
Thickness: 0.39mm
Second acoustic matching layer 25b
Density: 0.2 × 10 ^Three kg / m ^Three , Sound speed: 200m / s
Acoustic impedance: 4.0 × 10 ^Four kg / m ² / S
Thickness: 0.10mm
[0307]
The transmission / reception waveform of the ultrasonic transceiver according to the present embodiment is shown in FIG. In FIG. 27, the ultrasonic transmission / reception waveform of the ultrasonic transmission / reception unit of this embodiment is indicated by a solid line, and the transmission / reception waveform form of the ultrasonic transmission / reception unit (comparative example) in which the thicknesses of the acoustic matching layer and the protective matching layer are equal. Is indicated by a dotted line. As can be seen from FIG. 27, according to the ultrasonic transducer of this embodiment, high sensitivity can be realized.
[0308]
In the present embodiment, the acoustic matching layer 25 is composed of two layers. However, even if three or more layers are used, the same effect can be obtained by designing the upper surface portion of the protective matching layer 24 so that the phases of the ultrasonic waves are aligned. can get.
[0309]
Embodiment 12
A twelfth embodiment of the ultrasonic transducer according to the present invention will be described with reference to FIG. The characteristic point of this embodiment is that the first acoustic matching layer 25a and the protective matching layer 24 are integrally formed of the same material. A second acoustic matching layer 25b made of dry gel is formed on the first acoustic matching layer 25a.
[0310]
In the present embodiment, the sound velocity and wavelength of the ultrasonic waves in the first acoustic matching layer 25a and the protective matching layer 24 are equal, and the thickness L11 of the protective matching layer 24 is set to ¼ wavelength of the ultrasonic waves. For this reason, the thickness L8 of the first acoustic matching layer 25a is smaller than the 1/4 wavelength of the ultrasonic wave. The thickness L8 of the first acoustic matching layer 25a is determined by the thickness L9 of the second acoustic matching layer 25b and the distance L10 from the upper surface of the second acoustic matching layer 25b to the upper surface level of the protective matching layer 24.
[0311]
Also in the present embodiment, a phase delay of an integral multiple of the ultrasonic wavelength between the ultrasonic wave radiated through the acoustic matching layers 25a and 5b and the ultrasonic wave radiated through the protective matching layer 24 is obtained. Adopting a configuration that causes For this reason, at the upper surface level of the protective matching layer 24, the phases of the ultrasonic waves propagating through the acoustic matching layers 25a and 25b and the propagation medium 26 are aligned.
[0312]
In order to increase the sensitivity of the ultrasonic waves transmitted through the acoustic matching layers 25a and 25b, the thickness of the second acoustic matching layer 25b is more important than the thickness of the first acoustic matching layer 25a. In the present embodiment, the thickness of the second acoustic matching layer 25b is set to about ¼ of the wavelength of ultrasonic waves to be transmitted and received. The thickness of the first acoustic matching layer 25a also affects the sensitivity, but most of the influence extends to the frequency ratio band.
[0313]
For this reason, in this embodiment, the characteristic of the dry gel layer which can comprise the 2nd acoustic matching layer 25b is first determined from points, such as mechanical strength of a material. Next, the thickness L8 of the first acoustic matching layer 25a formed from the same material as the protective matching layer 24 and the thickness L10 of the acoustic wave propagation medium are set.
[0314]
In the present embodiment, the porous matching ceramic is used as the material of the protective matching layer 24 as in the above-described embodiment, and the thickness (L11) is set to ¼ of the ultrasonic wavelength. That is, L11 is set to 1.0 mm. In this case, the sound velocity of the first acoustic matching layer 25a formed from the porous ceramic is 200 m / s, and the density is 0.2 × 10. ^Three kg / m ^Three It becomes. In order to set the thickness of the second acoustic matching layer 25b formed from the dried gel to ¼ of the ultrasonic wavelength, L9 is set to 0.10 mm.
[0315]
At this time, the following Expression 7 is established.
[0316]
L8 + L10 = 0.9 [mm] (7)
[0317]
Equation 7 is derived from setting L11 to 1.0 mm and L9 to 0.1 mm.
[0318]
In order to exhibit excellent characteristics, it is preferable that the following expression holds.
[0319]
L8 / 1 + L10 / (17/25) = 1 [wavelength] (8)
[0320]
In the present embodiment, since ultrasonic waves having a frequency of 500 kHz are transmitted and received, one wavelength of the ultrasonic waves in the acoustic matching layer 25a is 1.0 mm, and one wavelength of the ultrasonic waves in the sound wave propagation medium 26 is 17/25 mm. Equation 8 is the sum of the ratio of the thickness L8 of the first acoustic matching layer 25a to one wavelength of the ultrasonic wave and the ratio of the thickness L10 of the propagation medium 26 to one wavelength of the ultrasonic wave. Satisfying Equation 8 means that the ultrasonic wave travels by one wavelength when passing through the first acoustic matching layer 25a and the sound wave propagation medium 26. In other words, it means that the effective thickness of the first acoustic matching layer 25a and the sound wave propagation medium 26 that the ultrasonic waves feel is equivalent to one wavelength.
[0321]
When L8 and L10 satisfying Expressions 7 and 8 are calculated, L8 = about 0.69 mm and L10 = 0.21 mm.
[0322]
Next, a method for manufacturing the ultrasonic transducer according to this embodiment will be described with reference to FIGS.
[0323]
First, as shown in FIG. 29A, a 1.0 mm thick pellet made of porous ceramics is prepared, and this pellet is processed as shown in FIG. 29B. In this embodiment, a groove is formed on the upper surface of the pellet, and the thickness of the groove bottom is adjusted to 0.69 mm. This groove bottom is a portion that functions as the first acoustic matching layer 25a. The groove is formed in a ring shape as shown in FIG.
[0324]
Next, as shown in FIG. 29C, the second acoustic matching layer 25b is formed inside the groove. The thickness of the acoustic matching layer 25b is set to 0.1 mm. The composite of the protective matching layer 24 / acoustic matching layers 25a and 25b is joined to the piezoelectric body 22 as shown in FIG. 29 (d) to form the ultrasonic transducer 21.
[0325]
The first acoustic matching layer 25b is a first gel using a liquid obtained by mixing tetramethoxysilane / ethanol / 0.05N aqueous ammonia at a molar ratio of 1/7/4 in the same manner as in the first embodiment. It is formed by performing a crystallization process.
[0326]
Adhesion of the composite composed of the protective matching layer 24 / acoustic matching layers 25a and 25b to the piezoelectric body can be performed with an epoxy adhesive, as in the first embodiment.
[0327]
According to this embodiment, since the protective matching layer 24 and the acoustic matching layer 25a can be collectively formed, the manufacturing process can be simplified and the manufacturing cost can be reduced.
[0328]
(Embodiment 13)
A thirteenth embodiment of an ultrasonic transducer according to the present invention will be described with reference to FIG. A characteristic point of this embodiment is that it has a structural support.
[0329]
The ultrasonic transducer of this embodiment is the ultrasonic wave of Embodiment 1 except that the structure support 27 is provided between the piezoelectric body 22 and the first acoustic matching layer 25 or the protective matching layer 24. It has the same configuration as that of the transmission / reception recording medium.
[0330]
The structural support 27 includes a disk-shaped support portion to which the acoustic matching layer 25 and the like are fixed, and a cylindrical portion that extends continuously from the disk-shaped support portion in the axial direction. The end surface of the cylindrical portion is bent in an L shape and is easily fixed to a plate (not shown) for shielding the piezoelectric body 22 or another device.
[0331]
The acoustic matching layer 25 and the protective matching layer 24 are disposed on the surface of the structural support 27, and the piezoelectric body 22 is disposed on the back surface of the support portion. By using such a structure support 27, the ultrasonic transducer can be handled very easily.
[0332]
The structural support can be composed of a container (sensor case) that can be sealed. In this case, if the open end of the cylindrical portion of the structural support 27 is closed with a shielding plate or the like, and the inside of the structural support 27 is filled with an inert gas, the piezoelectric body 22 is blocked from the fluid whose flow rate is to be measured. be able to.
[0333]
Since a voltage is applied to the piezoelectric body 22, there is a risk that the combustible gas may be ignited when the piezoelectric body comes into contact with the combustible gas. However, the structural support 27 is formed of a hermetically sealed container, and the inside of the piezoelectric body 22 is cut off from an external fluid or the like to prevent such ignition and to be safe against flammable gases. Sound waves can be transmitted and received.
[0334]
Even when not a combustible gas, the piezoelectric body 22 is shielded from external gas even when ultrasonic waves are transmitted to and received from a gas that reacts with the piezoelectric body 22 and may deteriorate the characteristics of the piezoelectric body 22. It is preferable to do. By doing so, it is possible to prevent the piezoelectric body 22 from deteriorating and realize a highly reliable operation over a long period of time.
[0335]
Of the structural support 27, the portion located between the piezoelectric body 22 and the acoustic matching layer 25 or the protective matching layer 24 does not function as an acoustic matching layer. Therefore, in order to prevent the structural support 27 from acting as an acoustic obstacle, the thickness of the portion of the structural support 27 that is located between the piezoelectric body 22 and the acoustic matching layer 25 or the protective matching layer 24. Is preferably about 1/8 or less of the wavelength of ultrasonic waves to be transmitted and received.
[0336]
In the present embodiment, the structure support 27 is made of stainless steel, and the thickness of the portion is set to 0.2 mm.
[0337]
The speed of sound of stainless steel is about 5500 m / second, and the wavelength of ultrasonic waves at 500 kHz is about 11 mm. Since the thickness of 0.2 mm corresponds to about 1/55 of the wavelength, the presence of the structural support 27 is hardly an acoustic impediment.
[0338]
The material of the structural support 27 is not limited to a metal material such as stainless steel, and a material corresponding to the purpose is selected from ceramic, glass, resin, and the like. In this embodiment, the external fluid and the piezoelectric body are reliably separated, and even if some mechanical impact is applied to the structural support body, the metal material is provided with a strength capable of preventing contact between the piezoelectric body and the external fluid. The structural support 27 is manufactured from the above. Thereby, for example, high safety can be ensured even when ultrasonic waves are transmitted and received for flammable or explosive gases.
[0339]
In addition, when performing ultrasonic transmission / reception with respect to safe gas, you may use the structure support body which consists of materials, such as resin, for the purpose of cost reduction.
[0340]
(Embodiment 14)
A fourteenth embodiment of an ultrasonic transducer according to the present invention will be described with reference to FIGS. 31 (a) and (b). FIGS. 31A and 31B are top views of the present embodiment.
[0341]
In the example shown in FIG. 21, a porous ceramic ring (three ring-shaped members having the same width and different diameters) functioning as the protective matching layer 24 is used and arranged on the main surface of the piezoelectric body so that their centers coincide. However, the protective matching layer 24 may be formed using rings having different widths as shown in FIG. Further, as shown in FIG. 31B, the island-shaped protective matching layers 24 may be randomly arranged.
[0342]
When the protective matching layer 24 and the acoustic matching layer 25 are regularly arranged on the main surface of the ultrasonic transducer, the phase of the ultrasonic wave is in a direction having a certain angle with respect to the main surface. As a result, the amplitude increases. This is called a “sidelobe” and becomes an obstruction factor when performing ultrasonic measurement. However, as shown in FIG. 31, by adopting a configuration in which the arrangement of the protective matching layer 24 does not have periodicity, it is possible to suppress side lobes and enable ultrasonic measurement with high accuracy and reliability. .
[0343]
(Embodiment 15)
A fifteenth embodiment of an ultrasonic transducer according to the present invention will be described with reference to FIG.
[0344]
The ultrasonic transducer according to this embodiment has a first characteristic point in that the thickness of the protective matching layer 24 has an in-plane distribution. In each of the above-described embodiments, the thickness of the protective matching layer 24 is set uniformly in the plane, but in this embodiment, an in-plane distribution is intentionally given. The second feature of the present embodiment is that the protective matching layer 24 provided on the piezoelectric body 22 is formed of two different materials.
[0345]
According to the configuration of the present embodiment, by using different materials and / or providing in-plane distributions of different thicknesses, side lobes can be suppressed, or the frequency of ultrasonic waves to be transmitted and received can be changed to broaden the bandwidth. Can do.
[0346]
The thickness of each protective matching layer 24 is preferably included in the range of 1/8 to 1/3 of the ultrasonic wavelength, and the range of 1/6 to 1/4 of the ultrasonic wavelength. More preferably, it is contained within. However, the thickness of a part of the protective matching layer 24 having a different thickness may be out of the above range. Since the protective matching layer 24 having a thickness outside the above range does not function as an acoustic matching layer, the sensitivity of ultrasonic transmission / reception decreases. However, by placing a protective layer that does not function as an acoustic matching layer (which can no longer be called a “protective matching layer”) at an appropriate position on the piezoelectric body, the disturbance of the ultrasonic field at a short distance can be prevented and a good super Sound wave measurement can be made possible.
[0347]
(Embodiment 16)
An embodiment of an ultrasonic flowmeter according to the present invention will be described with reference to FIG.
[0348]
The ultrasonic flowmeter of the present embodiment is installed so that the fluid to be measured flows at a velocity V in a pipe that functions as the flow rate measurement unit 51. On the tube wall 52 of the flow rate measuring unit 51, ultrasonic transmission receivers 1a and 1b formed from the ultrasonic transmitter / receiver of the present invention are arranged to face each other.
[0349]
At some point, the ultrasonic transmitter / receiver 1a functions as an ultrasonic transmitter and the ultrasonic transmitter / receiver 1b functions as an ultrasonic receiver. At other points, the ultrasonic transmitter / receiver 1a operates as an ultrasonic receiver. The ultrasonic transmitter / receiver 1b functions as an ultrasonic transmitter. This switching is performed by the switching circuit 53.
[0350]
The ultrasonic transceivers 1a and 1b are connected via a switching circuit 53 to a drive circuit 54 that drives the ultrasonic transceivers 1a and 1b and a reception detection circuit 55 that detects ultrasonic pulses. The output of the reception detection circuit 55 is sent to a timer 56 that measures the propagation time of the ultrasonic pulse. The output of the timer 56 is sent to a calculation unit 57 that calculates the flow rate. The computing unit 57 calculates the velocity V of the fluid flowing in the flow rate measuring unit 51 based on the measured propagation time of the ultrasonic pulse, and obtains the flow rate. The drive circuit 54 and the timer 56 are connected to the control unit 58 and controlled by a control signal output from the control unit 58.
[0351]
Hereinafter, the operation of this ultrasonic flowmeter will be described in more detail.
[0352]
Consider a case where LP gas flows through the flow rate measuring unit 51 as a fluid to be measured. The drive frequency of the ultrasonic transmission receivers 1a and 1b is about 500 kHz. The control unit 58 outputs a transmission start signal to the drive circuit 54 and starts time measurement of the timer 56 at the same time. When receiving the transmission start signal, the drive circuit 54 drives the ultrasonic transmission receiver 1a and transmits an ultrasonic pulse. The transmitted ultrasonic pulse propagates through the flow rate measurement unit 51 and is received by the ultrasonic transmission receiver 1b. The received ultrasonic pulse is converted into an electrical signal by the ultrasonic transmission receiver 1 b and output to the reception detection circuit 55.
[0353]
The reception detection circuit 55 determines the reception timing of the reception signal and stops the timer 56. The calculator 57 calculates the propagation time t1.
[0354]
Next, the switching circuit 53 switches the ultrasonic transmission receivers 1 a and 1 b connected to the drive circuit 54 and the reception detection circuit 55. Then, again, the control unit 59 outputs a transmission start signal to the drive circuit 54 and simultaneously starts the time measurement of the timer 56. Contrary to the measurement of the propagation time t1, an ultrasonic pulse is transmitted by the ultrasonic transmission receiver 1b, received by the ultrasonic transmission receiver 1a, and the propagation time t2 is calculated by the calculation unit 57.
[0355]
Here, the distance connecting the centers of the ultrasonic transmission receiver 1a and the supersonic transmission receiver 1b is L, the speed of sound in the absence of wind of LP gas is C, the flow velocity in the flow rate measurement unit 51 is V, The angle between the direction of the flow of the measurement fluid and the line connecting the centers of the ultrasonic transmission receivers 1a and 1b is θ.
[0356]
The propagation times t1 and t2 are each obtained by measurement. Since the distance L is known, the flow rate V can be obtained by measuring the times t1 and t2, and the flow rate can be determined from the flow rate V.
[0357]
In such an ultrasonic flowmeter, the propagation times t1 and t2 are measured by a method called a zero cross method. In this method, an appropriate threshold level is set for the received waveform as shown in FIG. 34 (a), and the time at which the amplitude becomes 0 after the threshold level is measured. When the S / N of the received signal is poor, the point at which the amplitude becomes 0 varies with time depending on the noise level, so t1 and t2 cannot be measured accurately, and it is difficult to measure the accurate flow rate. It may become.
[0358]
When the ultrasonic transmitter / receiver of the present invention is used as an ultrasonic transducer of such an ultrasonic flowmeter, the S / N of the received signal is improved, and t1 and t2 can be measured with high accuracy. .
[0359]
As shown in FIG. 34 (b), when the rising edge of the received signal is slower (narrow band) than in the case of FIG. 34 (a), t1 and t2 are measured with respect to the set value of the threshold level. The peak position of the received signal may fluctuate, resulting in measurement errors. However, since the ultrasonic transmitter / receiver according to the present invention operates appropriately in a wide band, the reception signal rises well, and accurate flow rate measurement can be stably performed. In addition, it is preferable to use the average value of the value obtained by the measurement of other times as a value of t1 and t2.
[0360]
The ability to transmit and receive broadband ultrasonic waves means that the signal falls early. For this reason, even when measurement is repeated quickly, it is not affected by the previous transmission / reception signal. As a result, even if the repetition frequency of measurement is increased, instantaneous measurement is possible, and gas leaks can be detected instantaneously.
[0361]
In the above embodiments, the upper surface of the uppermost acoustic matching layer (first acoustic matching layer) is exposed, but this surface may be covered with a protective film having a thickness of 10 μm or less. Such a protective film avoids direct contact between the atmosphere and the acoustic matching layer, and contributes to maintaining the characteristics of the acoustic matching layer over a long period of time. The protective film is constituted by a film (not limited to a single layer) made of a material such as aluminum, silicon oxide, low-melting glass, or polymer. The protective film is deposited by sputtering or CVD.
[0362]
[Brief description of the drawings]
[0363]
FIG. 1 is a cross-sectional view showing an ultrasonic transceiver according to a first embodiment of the present invention.
FIG. 2 is a top view of the ultrasonic transceiver according to the first embodiment of the present invention.
3A is a graph showing a transmission / reception waveform of the ultrasonic transducer according to the first embodiment of the present invention, and FIG. 3B is a graph showing a transmission / reception waveform of a conventional ultrasonic transducer. .
FIG. 4 is a cross-sectional view schematically showing a case where an acoustic matching layer contracts in Embodiment 1 of the present invention.
FIG. 5 is a cross-sectional view of an ultrasonic transceiver according to a second embodiment of the present invention.
FIG. 6 is a cross-sectional view showing another configuration of the protection unit according to the second embodiment of the present invention.
FIG. 7 is a top view illustrating another configuration of the protection unit according to the second embodiment of the present invention.
FIG. 8 is a cross-sectional view of an ultrasonic transceiver according to a third embodiment of the present invention.
FIG. 9 is a cross-sectional view of an ultrasonic transceiver according to a fourth embodiment of the present invention.
FIG. 10 is a cross-sectional view of an ultrasonic transceiver according to a fifth embodiment of the present invention.
FIG. 11A is a graph showing a transmission / reception waveform of an ultrasonic transducer according to a fifth embodiment of the present invention, and FIG. 11B is a graph showing a transmission / reception waveform of a conventional ultrasonic transducer. .
FIG. 12 is a cross-sectional view showing another configuration of the lower acoustic matching layer according to Embodiment 6 of the present invention.
FIG. 13 is a cross-sectional view of an ultrasonic transceiver according to a seventh embodiment of the present invention.
FIG. 14 is a graph showing a transmission / reception waveform of the ultrasonic transducer according to the seventh embodiment of the present invention.
FIG. 15 is a cross-sectional view of an ultrasonic transceiver with another configuration according to Embodiment 7 of the present invention.
16 (a) to 16 (c) are process cross-sectional views illustrating a method for manufacturing the ultrasonic transducer shown in FIG.
17A is a cross-sectional view showing an acoustic matching layer when gel permeation is sufficient in the manufacturing process shown in FIG. 16, and FIG. 17B is an acoustic diagram when gel permeation is insufficient. It is sectional drawing which shows a matching layer.
18A to 18D are process cross-sectional views showing another method for manufacturing the ultrasonic transducer shown in FIG.
FIGS. 19A and 19B are top views showing other configuration examples of the protection unit, respectively.
FIG. 20 is a sectional view of a tenth embodiment of an ultrasonic transducer according to the present invention.
FIG. 21 is a top view of a tenth embodiment of an ultrasonic transducer according to the present invention.
FIG. 22 is a schematic diagram showing ultrasonic interference in the tenth embodiment of the ultrasonic transducer according to the present invention;
FIG. 23 is a cross-sectional view schematically showing the phase of an ultrasonic wave propagated through the protective matching layer and the acoustic matching layer.
24 (a) to 24 (c) are process cross-sectional views illustrating a method for manufacturing a tenth embodiment of an ultrasonic transducer according to the present invention.
FIG. 25 is a transmission / reception waveform diagram of the tenth embodiment of the ultrasonic transducer according to the present invention;
FIG. 26 is a sectional view of an eleventh embodiment of an ultrasonic transducer according to the present invention.
FIG. 27 is a transmission / reception waveform diagram of the eleventh embodiment of the ultrasonic transducer according to the present invention;
FIG. 28 is a sectional view of a twelfth embodiment of an ultrasonic transducer according to the present invention.
29 (a) to 29 (d) are process cross-sectional views illustrating a method for manufacturing a twelfth embodiment of an ultrasonic transducer according to the present invention.
FIG. 30 is a sectional view of a thirteenth embodiment of an ultrasonic transducer according to the present invention.
FIGS. 31 (a) and 31 (b) are top views of a fourteenth embodiment of an ultrasonic transducer according to the present invention, respectively.
FIG. 32 is a cross-sectional view of an ultrasonic transducer according to a fifteenth embodiment of the present invention.
FIG. 33 is a block diagram illustrating an ultrasonic flowmeter according to a sixteenth embodiment of the present invention.
34 (a) and 34 (b) are graphs showing waveforms measured by the ultrasonic flowmeter of the present invention.
FIG. 35 is a cross-sectional view showing a conventional ultrasonic flowmeter.
FIG. 36 is a cross-sectional view of a conventional ultrasonic transceiver.

Claims (56)

A piezoelectric body;
An acoustic matching layer provided on the piezoelectric body;
A protective part that is in contact with at least a part of the side surface of the acoustic matching layer and is fixed to the piezoelectric body;
Ultrasonic transceiver with The protective portion protrudes in the ultrasonic radiation direction from the same level as the main surface of the piezoelectric body, and the height of the protective portion with respect to the main surface of the piezoelectric body is the thickness of the acoustic matching layer. The ultrasonic transceiver according to claim 1, wherein The ultrasonic transceiver according to claim 1, wherein the height of the protection unit is 5 μm or more and 2500 μm or less. The ultrasonic transceiver according to claim 3, wherein a thickness of the acoustic matching layer is substantially equal to the height of the protection unit. 5. The ultrasonic transceiver according to claim 1, wherein a thickness of the acoustic matching layer is about ¼ of a wavelength of an ultrasonic wave transmitted and / or received by the piezoelectric body. The ultrasonic transceiver according to claim 1, wherein the acoustic matching layer is formed of a material having a density of 50 kg / m ³ or more and 1000 kg / m ³ or less. The acoustic matching layer is formed of a material having an acoustic impedance of 2.5 × 10 ³ kg / m ² / s or more and 1.0 × 10 ⁶ kg / m ² / s or less. The ultrasonic transceiver described in 1. The ultrasonic transceiver according to claim 6 or 7, wherein the acoustic matching layer is formed of an inorganic material. The ultrasonic transceiver according to claim 8, wherein the inorganic material is a dry gel of an inorganic oxide. The ultrasonic transceiver according to claim 9, wherein the inorganic oxide has a water-repellent solid skeleton. 11. The ultrasonic transceiver according to claim 1, wherein the acoustic matching layer is solidified from a fluid state on the piezoelectric body provided with the protection unit. A lower acoustic matching layer provided between the main surface of the piezoelectric body and the acoustic matching layer;
The protective part protrudes from the main surface of the lower acoustic matching layer, and the height of the protective part with respect to the main surface of the acoustic matching layer defines the thickness of the acoustic matching layer positioned at the uppermost layer The ultrasonic transceiver according to claim 1. The ultrasonic transceiver according to claim 12, wherein the protection unit is configured by a part of the lower acoustic matching layer and is integrated with the lower acoustic matching layer. The ultrasonic transmitter / receiver according to claim 12 or 13, wherein the height of the protection part is not less than 5 µm and not more than 2500 µm. The ultrasonic transceiver according to claim 14, wherein the height of the protection unit is substantially equal to a thickness of the acoustic matching layer located in an uppermost layer. The super acoustic matching layer according to any one of claims 12 to 15, wherein each of the acoustic matching layer and the lower acoustic matching layer has a thickness of about 1/4 of a wavelength of an ultrasonic wave transmitted and received by the piezoelectric body. Sonic transceiver. The ultrasonic transceiver according to any one of claims 12 to 16, wherein the density of the acoustic matching layer is 50 kg / m ³ or more and 1000 kg / m ³ or less. The acoustic impedance of the lower acoustic matching layer is greater than the acoustic impedance of the first acoustic matching layer, and is 2.5 × 10 ³ kg / m ² / s or more and 3.0 × 10 ⁷ kg / m ² / s or less. The ultrasonic transceiver according to any one of claims 12 to 17. The ultrasonic transmitter / receiver according to any one of claims 1 to 18, wherein the protection unit is present on an outer periphery of the acoustic matching layer located in an uppermost layer. The ultrasonic transmitter / receiver according to claim 19, wherein the protection unit covers the entire outer peripheral side surface of the acoustic matching layer located in the uppermost layer. The ultrasonic transmitter / receiver according to claim 19 or 20, wherein the protection unit is disposed outside a main surface of the piezoelectric body. The ultrasonic transceiver according to claim 1, wherein the protection unit is provided on a main surface of the piezoelectric body. The ultrasonic transceiver according to claim 12, wherein the protection unit is provided on the lower acoustic matching layer. The ultrasonic transmitter / receiver according to claim 1, wherein the protection unit is configured by a part of the piezoelectric body and is integrated with the piezoelectric body. The ultrasonic transceiver according to any one of claims 1 to 24, further comprising a structural support for supporting the piezoelectric body. A structural support for supporting the piezoelectric body;
The ultrasonic transceiver according to claim 1, wherein the protection part is provided on the structural support. The structural support is formed from a press-molded metal;
27. The ultrasonic transceiver according to claim 26, wherein the protection part is configured by a portion bent by press molding of the structural support. The ultrasonic transmitter / receiver according to claim 26 or 27, wherein the height of the protection part is not less than 5 µm and not more than 2500 µm. 29. The ultrasonic transceiver according to claim 28, wherein a thickness of the acoustic matching layer is substantially equal to the height of the protection unit. 30. The ultrasonic transmitter / receiver according to claim 28 or 29, wherein a thickness of the acoustic matching layer is about 1/4 of a wavelength of an ultrasonic wave transmitted and received by the piezoelectric body. The ultrasonic transceiver according to any one of claims 26 to 30, wherein the density of the acoustic matching layer is 50 kg / m ³ or more and 1000 kg / m ³ or less. 31. The ultrasonic wave according to claim 25, wherein an acoustic impedance of the acoustic matching layer is 2.5 × 10 ³ kg / m ² / s or more and 1.0 × 10 ⁶ kg / m ² / s or less. Transceiver. A back load material that is disposed on the back surface of the piezoelectric body and attenuates ultrasonic waves emitted from the piezoelectric body in the back direction;
The ultrasonic transceiver according to claim 1, wherein the protection member is provided on the back load material. 34. The ultrasonic transceiver according to claim 33, wherein the protection unit is configured by a part of the back surface load material and is integrated with the back surface load material. The ultrasonic transceiver according to any one of claims 1 to 34, wherein at least a part of a surface of the acoustic matching layer in contact with the holding portion is subjected to a surface treatment for imparting a hydroxyl group. The ultrasonic transceiver according to any one of claims 1 to 34, wherein at least a part of a surface where the acoustic matching layer is in contact with the holding unit is subjected to a roughening treatment. The ultrasonic transmitter / receiver according to any one of claims 1 to 34, wherein at least a part of a surface of the acoustic matching layer in contact with the holding unit is porous. The ultrasonic transmitter / receiver according to any one of claims 1 to 34, wherein a part of the acoustic matching layer penetrates and is integrated with a portion of the ultrasonic transmitter / receiver in contact with the acoustic matching layer. A piezoelectric body that performs ultrasonic vibration;
It is formed from a material having a density of 50 kg / m ³ or more and 1000 kg / m ³ or less and an acoustic impedance of 2.5 × 10 ³ kg / m ² / s or more and 1.0 × 10 ⁶ kg / m ² / s or less. An upper acoustic matching layer;
A lower acoustic matching layer provided between the piezoelectric body and the upper acoustic matching layer;
A structural support that supports the lower acoustic matching layer and the piezoelectric body, and shields the piezoelectric body from ultrasonic propagation fluid;
An ultrasonic transceiver comprising:
An ultrasonic transmitter / receiver comprising a protection unit that contacts at least a part of a side surface of the upper acoustic matching layer. The ultrasonic transceiver according to claim 39, wherein the protection part is formed by a part of the lower acoustic matching layer and is integrated with the lower acoustic matching layer. 41. The ultrasonic transceiver according to any one of claims 1 to 40, wherein an elastic modulus of the protection part is substantially equal to an elastic modulus of the acoustic matching layer. A flow rate measurement unit through which the fluid to be measured flows;
A pair of ultrasonic transmitters / receivers provided in the flow rate measurement unit for transmitting / receiving ultrasonic signals;
Measuring means for measuring the time for ultrasonic waves to propagate between the pair of ultrasonic transceivers;
An ultrasonic flowmeter comprising flow rate calculation means for calculating a flow rate based on a signal from the measurement means,
An ultrasonic flowmeter, wherein each of the pair of ultrasonic transceivers is the ultrasonic transceiver according to any one of claims 1 to 41. The ultrasonic flowmeter according to claim 42, wherein a piezoelectric body of the ultrasonic transceiver is shielded from the fluid to be measured. The apparatus provided with the ultrasonic transmitter-receiver in any one of Claims 1-41. A step (a) of preparing a piezoelectric body having a main surface and a convex portion received on the main surface;
Forming an acoustic matching layer on the main surface of the piezoelectric body, and contacting at least a part of the side surface of the acoustic matching layer with the side surface of the convex portion;
A method for manufacturing an ultrasonic transmitter-receiver. The step (b) includes a step of supplying a gel raw material onto the main surface of the piezoelectric element;
Forming the acoustic matching layer by drying and solidifying the gel raw material;
The manufacturing method of Claim 45 containing these. The said process (a) is a manufacturing method of Claim 45 including the process of processing the surface of a piezoelectric material and forming the said main surface and a convex part. The manufacturing method according to claim 45, wherein the step (a) includes a step of fixing the convex portion to a surface of the piezoelectric body. The manufacturing method according to claim 45, wherein the step (a) includes a step of fixing the piezoelectric body to the structural support. A method of manufacturing an ultrasonic transceiver including an upper acoustic matching layer, a piezoelectric body, and a lower acoustic matching layer provided between the upper acoustic matching layer and the piezoelectric body,
A step (a) of preparing a member having a recess and functioning as the lower acoustic matching layer;
Supplying a gel raw material to the recess of the member (b);
(C) forming the upper acoustic matching layer by drying and solidifying the gel raw material;
Including methods. 51. The manufacturing method according to claim 50, wherein the step (b) includes a step of infiltrating the gel raw material into the member. 52. The manufacturing method according to claim 51, wherein the gel raw material is permeated throughout the member. 51. The manufacturing method according to claim 50, wherein the step (b) is performed after fixing a positional relationship between the member and the piezoelectric body. 51. The manufacturing method according to claim 50, wherein the step (b) is performed before fixing a positional relationship between the member and the piezoelectric body. A piezoelectric body;
An acoustic matching layer provided on the piezoelectric body;
A protective part arranged to contact the outer peripheral surface of the acoustic matching layer;
Ultrasonic transceiver with A structural support;
A piezoelectric body and an acoustic matching layer provided at positions facing each other across the structural support;
A protective part arranged to contact the outer peripheral surface of the acoustic matching layer;
Ultrasonic transceiver with

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